[suboptimal]

"every year he tops out at about 60% of his time being billable"

Another general guideline is to estimate 1,000 hours of billable work (which makes it easy to calculate an hourly rate for some amount of desired annual earnings).

"Also, don't forget taxes."

Definitely don't forget the taxes, especially estimated payments. Consider that 40% of that money, once deposited, isn't yours and is untouchable, and you'll be better off.