If you consider a sagged fork (as you’re riding it) – the angles aren’t affected as much as that. stock fork is 470mm, a 120mm pike is 531mm unsagged, or 490mm sagged @ 30%. 20mm increase in a2c would change the angles by approx 1 deg (so 67 head angle and 74deg seat).
I’d stick to a 120mm (maybe 130mm) fork anyway..
Ya this is a good point, I gave unsagged numbers so you can compare with other bike’s geometry, which are normally quoted without sag.
As it happens my numbers for a 100mm fork weren’t far off anyway as you have to add an external headset like someone else said, which adds close to another 15mm I think?