Right now the highest is around 7,000 inborn skills usually that is plus a tear during a female cover so you definitely got a high inborn of 6,400.

I'm pretty sure this formula is still correct for calculating the highest possible IBS, but I'm no longer sure what variable the lowest is as I know there was a change a while back.
Assuming both parents have 100 BLUP - Add both parents GP together, then divide by 10. If using Aphrodite Tears, then times by 1.5, and finally times by 1.04 to see the highest variable.

For example: The current highest horse is 22,900 GP, so I'll use that value. 22,900 + 22,900 = 45,800. Divide by 10 = 4,580. Times 1.5 = 6870. Times 1.04 means 7144.8 is the maximum inborn skills a horse of this GP can get.

If this has changed, someone please let me know

Somehow I have a "times 1.06" for the maximum in my formula. I can't remember why. All I seem to remember is there being some update "recently", where something was said about GP. But I don't find it. :/

Ah, maybe the difference was because of the "+50% inborn skills" of AT. 4% variance + 50% with 4% variance are 4%+1/2*4% = 6%. That's where those came from.

Like this:
Let's say the inborn skills are at an average of 100. AT adds 50% to this, so it'd be 150.
100-(4%*100) = 100-4 = 96. If the bonus of AT is applied afterwards (I don't know how this works exactly): 96 + 0.5 * 96 = 96 + 48 = 144.
100+(4%*100) = 100+4 = 104. 104+0.5*104 = 104 + 52 = 156.
So it's a difference of 6 in any direction. That's 6% of the initial 100, but 4% of the skills with AT.
It depends on where you put the "times 1.0x"

Solved it, sorry for the potential confusion ^^

For a GP of 22900 I ended up with 7144.8 too ^^