In her much-discussed new book, Facebook Chief Operating Officer Sheryl Sandberg tells women with high aspirations that they need to 'lean in' at work -- that is, assert themselves more. It's fine advice, but it misdiagnoses the problem. It isn't any shortage of drive that leads those phalanxes of female Harvard Business School grads to opt out. It's the assumption that senior roles have to consume their every waking moment. More great women don't 'lean in' because they don't like the world they're being asked to lean into.

It doesn't have to be this way. A little organizational imagination bolstered by a commitment from the C-suite can point the path to a saner, more satisfying blend of the things that ambitious women want from work and life. It's time that we put the clock at the heart of this debate.

I know this is doable because I run a growing startup company in which more than half the professionals work fewer than 40 hours a week by choice. They are alumnae of top schools and firms like General Electric and McKinsey, and they are mostly women. The key is that we design jobs to enable people to contribute at varying levels of time commitment while still meeting our overall goals for the company.

This isn't advanced physics, but it does mean thinking through the math of how work in a company adds up. It's also an iterative process; we hardly get it right every time. But for businesses and reformers serious about cracking the real glass ceiling for women -- and making their firms magnets for the huge swath of American talent now sitting on the sidelines -- here are four ways to start going about it.

Rethink time. Break away from the arbitrary notion that high-level work can be done only by people who work 10 or more hours a day, five or more days a week, 12 months a year. Why not just three days a week, or six hours a day, or 10 months a year?

It sounds simple, but the only thing that matters is quantifying the work that needs to get done and having the right set of resources in place to do it. Senior roles should actually be easier to reimagine in this way because highly paid people have the ability and, often, the desire to give up some income in order to work less. Flexibility and working from home can soften the blow, of course, but they don't solve the overall time problem.

Break work into projects. Once work is quantified, it must be broken up into discrete parts to allow for varying time commitments. Instead of thinking in terms of broad functions like the head of marketing, finance, corporate development or sales, a firm needs to define key roles in terms of specific, measurable tasks.

Once you think of work as a series of projects, it's easy to see how people can tailor how much to take on. The growth of consulting and outsourcing came precisely when firms realized they could carve work into projects that could be done more effectively outside. The next step is to design internal roles in smaller bites, too. An experienced marketer for a pharma company could lead one major drug launch, for example, without having to oversee all drug launches. Instead of managing a portfolio with 10 products, a senior person could manage five. If a client-service executive working five days a week has a quota of 10 deals a month, then one who chooses to work three days a week has a quota of only six. Lower the quota but not the quality of the work or the executive's seniority.

One reason this doesn't happen more is managerial laziness: It's easier to find a 'superwoman' to lead marketing (someone who will work as long as humanly possible) than it is to design work around discrete projects. But even superwoman has a limit, and when she hits it, organizations adjust by breaking up jobs and adding staff. Why not do this before people hit the wall?

Availability matters. It's important to differentiate between availability and absolute time commitments. Many professional women would happily agree to check email even seven days a week and jump in, if necessary, for intense project stints -- so long as over the course of a year, the time devoted to work is more limited. Managers need to be clear about what's needed: 24/7 availability is not the same thing as a 24/7 workload.

Quality is the goal, not quantity. Leaders need to create a culture in which talented people are judged not by the quantity of their work, but by the quality of their contributions. This can't be hollow blather. Someone who works 20 hours a week and who delivers exceptional results on a pro rata basis should be eligible for promotions and viewed as a top performer. American corporations need to get rid of the notion that wanting to work less makes someone a 'B player.'

Promoting this kind of innovation, where companies start to look more like puzzles than pyramids, has to become part of feminism's new agenda. It's the only way to give millions of capable women the ability to recalibrate the time that they devote to work at different stages of their lives.

We have been putting smart women on the couch for 40 years, since psychologist Matina Horner published her famous studies on 'fear of success.' But the portion of top jobs that go to women is still shockingly low. That's the irony of Ms. Sandberg's cheerleading for women to stay ambitious: She fails to see that her own agenda isn't nearly ambitious enough.

'Leaning in' may help the relative handful of talented women who can live with the way that top jobs are structured today -- and if that's their choice, more power to them. But only a small percentage of women will choose this route. Until the rest of us get serious about altering the way work gets done in American corporations, we're destined to howl at the moon over the injustice of it all while changing almost nothing.