Kind of relies on getting lucky with perk draws, but if you get a good pregnancy build, it seems to cap out. You'll get the cum-on-7 perk, multiple chance-of-pregnancy (it seems 100%+20% has a 20% of getting twins in one go), and 3-free-on-impreg, and then max out as soon as you finish cumming. I think it might be good to let free cum overflow, and if you have twice as much as needed, it doubles your cum effects.