I am pretty anal about mixing! Lol! I don't think I've ever undermixed anything!

You need to mix more thoroughly. 2-3 minutes stirring for a 500ml cup. 

Well I "used" to thin with Acetone, but had an incident where the gelcoat only hardened in patches, leaving some areas uncured. After that I went over to Styrene.
I also found that the Acetone seemed to evaporate out of the mixed gelcoat quite quickly as well, giving a viscosity change fairly quickly.

I have upped the MEKP ratio to 3% now and it seems a little better?

Hi Alex,

Thats a good question. Some of the extra styrene will crosslink, most of it will evaporate. So its hard to estimate catalization levels. I doubt it makes much difference at 15% A temperature change of 3 degrees probably has more effect. I would instead thin with acetone and catalyze as normal, not taking it into account. Acetone wont mess up with the crosslinking chemistry thats going on in the resin and the practical benefit is that most of it evaporates as it leaves the tip of the gun, so it lays down thick.

I have experimented with thinning GC50 with Styrene beyond the recommended 15%
I have gone to 30% and it still cures, albeit it's beginning to get a bit dubious by that point. The cured GC50 does not cure with a even gloss finish, sometimes all of the area is matt, sometimes patches. This doesn't matter too much as when the part is pulled from the mould it is usually still glossy, and any dull areas polish up great and don't degrade with time.

The gelling time though increases. My question is, and this applies to a more normal 15% thinning........ should I be accounting for the Styrene in my calculation for the MEKP?
To clarify, should I add the Styrene to the GC50 and calculate on the total?