One common technique used by agile methodologies (yes, I know that word has a ton of baggage) is to estimate in terms of story points rather than actual hours,

One common technique used by agile methodologies (yes, I know that word has a ton of baggage) is to estimate in terms of story points rather than actual hours, days, etc. You start out thinking of a point as "one ideal day of uninterrupted work" to use as an anchoring point, and then for any work you want to do you break that work down into stories that you can estimate work for. If the story is bigger than about 3 points, you break it down further; if you can't estimate it, you do more research so you can. The important part is not that the estimates match any number of hours per se, but rather than the relative weighting is about right, such that something you estimated as 3 points is, in actuality, about 3 times as much work as something estimated at 1 point, and all 1 point stories are roughly comparable in terms of work. What you then do is to empirically estimate the actual mapping of points to hours/days as you work by tracking your velocity. In a 40-hour work week, how many points do you actually get done? As you get better at estimating, ideally your velocity becomes more stable, and then you can map from points back to days in order to make scheduling decisions. We switched over project planning and estimation to that strategy on a fairly large project I was leading, and it helped give us much more accurate overall estimates. Now, that's all easier to do when you're working on a long-term project, you're familiar with the existing code base and the problem domain, etc. such that you can actually break things down into stories and do reasonable estimates. That said, I think that the general idea of point-based estimation is still useful for you: rather than trying to estimate hours or days, work on getting better at relative estimation using something like points, and then empirically measure your mapping from points to hours to determine your likely schedule and how much you should bill.