Derevka explains exactly what you're looking for in his Disc guide:
http://howtopriest.com/viewtopic.php?f=20&t=1837
"You will notice, that in some cases and varying gear levels, increasing secondary stats (at the 2:1 ratio) may be superior to that of Primary Stats. (ie. Increasing 200 Crit being superior to 100 Intellect). Evaluating your stats with Adinne's HealCalcis one way of determining those for you." - From Derevka's guide.
You will mainly gem Misty's, Potent's & Smooth gems this tier but i suggest you use the calculator mentioned above. I only changed to full crit once i reached a certain lvl of SP - around 40k but Adinne's Healcalc will give ya more accurate info for your play style & gear etc.