I will let everyone start with 70% of their die roll max plus whatever their constitution bonuses are.
If you are so inclined, I can roll for you but keep in mind it is one time and statistically you will probably hit closer to 50% of your die max.
Subsequent level hitpoints will be determined similarly except that we can roll in front of the dm and everyone or just choose your 70%.
If you opt for 70% it is still a flat 70%. So a fighter with a 10d HD at level 4 would be 40 × .7 = 28 + con bonus.
If you choose to have me roll for you it will be max hitpoints first level plus whatever I roll so the same fighter would be 10 + roll + roll + roll + Con bonus