Question about damage cutoff and IV.

It depends on the opponent.

If you are facing a perfect Blissey at lvl39, and your Tyranitar has perfect attack IV, then you will get maximum damage into Blissey from Bite on Tyranitar at lvl36.5

Change one variable...and the level required will change too.

So if the Blissey is weaker...the level needed is lower.
If the T-Rex att IV is not 15, then the level needs to be higher.

NB Blissey has fairly strong defence, and so most opponents will have lower defence, and so you can achieve max hit from Bite, without perfect IV long before lvl36.5 against certain defenders, so you may prefer to use a different lvl.

...basically, a calculator so we can place our mon with our personal IV in against a full range of selectable foes would be soooo handy in the current meta.
We could use it to fine tune both our attackers and defenders.

...and would bring a lot of traffic to GP

Use this to play more: https://docs.google.com/spreadsheets/d/1-ZSO1BciFhJaIvUHVOA3vv1uY2cXCV5JNGjOb6XpcX8/copy?usp=sharing

But in short:
vs. Moltres: you are lucky, Bite breakpoint is at lvl 24.5 with your attack, will deal 5 damage
vs. Zapdos: same luck, but at 26.5, dealing the same 5 damage
vs. Lugia, still lucky, lvl 21.5, dealing 4 damage
vs. Charizard: lvl 23
vs. Arcanine: 27.5
vs. Flareon: 26
vs. Alazazam: lvl 27 (dealing a substantial 7 damage)

For you case it might worth looking at defensive breakpoints vs, raid Boss quick move, since at lvl 30 with 11 attack you are set vs. any raid boss that has Tyranitar among best counters.