Cap Stats On Items - Help

Ok, someone, please give me a hand here... w-t-f!
So i have Synthesised a full HQ Vanya set (Yes, including the ridiculously expensive pants).
Now, I bought Rank IV INT Materias for 89k to discover i can't meld them into ANY of my Vanya set pieces. Seriously, wtf is that...
So i've been told every item has a cap of stats, but how am i suppose to know what is it and how to avoid purchasing costy materias in the future.
Meanwhile, im stuck with x4 IV's INT materias which i cannot meld at all for this set...
How do i know it wont happen with another tier IV materia i will purchase now?

I use a little 'trick': I have Tier 1 and Tier 2 of material (which is cheap) and try those. If they will not give me the full potential, then I do not even look for Tier 3 or 4. If they do, then I buy the next Tier and try that, and so on. Worst case scenario is that I get the max benefit of Tier 3, which happens to also be the cap limit, so I am 'stuck' with only 1 Tier 4 which I can try to use next time.

Oh, I could be wrong (probably am) but I think HQ items are maxed on the stats that they display. So if it shows INT+x or MND+x, that's as high as it will go. At least my experience so far.