If I zero it at 25 yards, how high will it shoot at 100?

I have not been able to get to a place where I can shoot 100 yet, so it has a 25 yard zero. A waste in a way, but I wanted to dial it in for something

zero it at 100 when you get there.  There will be a lot of fine tuning to do at that point.

Dunno, low couple inches. Not to be flippant, but you'd want to confirm zero at range before shooting that far anyway so it's a thought exercise more than anything. I'd bet depending on ammo your 25m zero would put you on again at somewhere between 200/300ish, and high on between.

ETA: Doesn't the rear sight adjust for elevation? I have a 17 but don't have it handy (I've always just used an ACOG so I'm not that familiar with the BUIS). If so I'd just shoot it at 25 with the aperture set to 300, and call it close enough until I could confirm at range.

What load?

If you zero at 25yds you'll be like 6" high at 100.

I have the Vortex Razor HDII 1-6 zeroed at 100yards on my 17s and know the drops. Irons are zeroed at 100 as well.