Are the to-hit estimates supposed to be roughly equal to the AC values -10? If that's the case, then the answer to your specific question would be 14. A party of four level 10's hitting at +15 and dealing 35 dpr at a 100% hit rate will, instead, deal about 42 damage per round to a creature with AC 30, which goes through those 112 HP in 3 rounds.
Also, a party of four level 20's hitting at +27 and dealing 70 dpr at a 100% hit rate will deal about 84 damage per round against a target with AC 42, which goes through those 192 HP in 3 rounds, but it's a lot closer to 2. You might want to plan to have HP grow slightly faster as levels go up.
Amusingly, this kinda looks like the way that 3e's math was SUPPOSED to work, since in 3 rounds, something 4 levels up should kill one of the four adventurers.