For those who have high earning potential and lack skills that are especially rare in the philanthropic world the most efficient way of helping people will typically be taking a high paying job and donating one's income to an efficient charity. Of course, many people who volunteer or forgo income to work at a non-profit do so not only with a view toward helping people but also because they want to experience the visceral sense of helping people directly or of working directly on a cause that they feel passionate about.

There's also the issue of doing no harm. I work in commercial litigation, at a "some-profit" job. My salary allows me a surplus to donate significant money to charity; I could instead choose to work at a "nonprofit job" and donate very little, or to work at a "for-profit" job and donate much more.

I chose the "some-profit job" over the non-profit job specifically because of the reasoning you cite -- I can probably do more good by donating the extra money than by being more helpful as a lawyer.

Why didn't I go all the way, and work at a for-profit job? Partly because I find the tasks and people associated with those firms obnoxious, but partly because they do a lot of harm and a lot of lying and a lot of cheating. Admittedly, if I were to shut up and multiply, I would still be saving net lives if I switched, at least if you only count the direct impact of what I do. But I don't feel comfortable limiting myself to direct impacts -- I do not know what the long-term, indirect impacts are of helping to perpetuate a system of lies and injustice and subtle economic oppression, nor do I know how to calculate them.

Advice is welcome, but please, tread carefully. Mere exhortations to "shut up and multiply anyway" are unlikely to move me.