I generally provide proposals in phases with one of them being specifically called out as CAD. This is where I throw out a number based on how many hours I would expect something like this to take if it were very simple and straight-forward, then multiply that by 50% because there will ALWAYS be issues that come up requiring creative solutions along the way. That’s what you offer as compared to some CAD guru straight out of mechanical engineering school. I provide this phase as a fixed cost but it’s based on having a very detailed scope determined first. If things deviate from that based on client requests, that’s when you start either providing them fixed costs for those changes or charge hourly.
Having been on both sides of the table when it comes to ID work, I agree that proposals need to confine total costs to a fixed range as long as client doesn’t make wild changes along the way. There will be changes, of course, as that’s just part of the process. The goal is to capture those expected fluctuations in development in your fixed cost so there are no surprises. Good way to lose a client is to give them a bill for extra work simply because you didn’t quote appropriately.
And when it comes to taxes, just expect to take out half and set it aside. This can be used for SEP 401K plans, Roths, etc. For the most part, if you keep this amount in the bank and only pay yourself 50% of your actual income, Uncle Sam won’t be knocking on your door for more.