If I make a promise to my team that I can reasonably keep, I owe it to them to do so.
I'd generally aim to under-promise and over-deliver, while feeling like I'm making a comprable (or bigger!) contribution in comparison to my peers. After some practice at this, I got reasonably good at estimating work. I'd work 20 to 50 hours per week depending on how accurately I estimate, usually aiming for (an achieving) about 35 hours of work. Only once or twice did I ever feel like I really put in any serious "overtime" and I blame that on estimation inexperience.
I made it a point to explain this philosophy (sans actual target hours) to every manager I've ever had. I always fed them the "Work/Life Balance" party line and reminded them that if I wanted my work to be my life, I'd join a startup (I have since founded one). They all seemed to appreciate my being forthcoming.
Once or twice I got a panicked email. The team was going to miss a deadline unless I stepped up to help out! Each time I replied that I had expressed my concerns about scope and timeline during the planning meetings. I'd remind the panicked person that we could simply cut the feature (always an option for a previously shipped product) and would offer my help and time in doing so. No one ever took me up on it.
> I made it a point to explain this philosophy (sans actual target hours) to every manager I've ever had.
I simply didn't tell my manager exactly how many hours I targeted because it was less than the customary 40h/wk.