Assuming I complete STS as devised, I would do week 1 of meso 1 as my test run. I'll grab weights based on what I've done previously for that exercise and see how it goes. I'll note if I need to go up or down for the "true" week 1. Instead of 12 weeks, it'll take 13.
The problem I see with doing 1 rep testing much before you start STS is your strength capacity could improve/worsen based on lots of factors (improved muscle development, illness, injury, overtraining). Others have certainly posted about this already. Furthermore, several of those who tried it said it was exhausting and screwed up their regular workouts.
I'm sure you could get better results if you did it, but how much better if you are already doing Cathe or similar level weight work? P90x doesn't have you do 1 rep max testing and that was a similarly conceived program. I figure my approach will be close enough. JMHO.