when melding more then 1 item, HQ will increase %'s as well as using the lower # materia first .. for example..

STR III +12 for first meld.. then second would be STR III +13.. this would give you a higher % then going 13>> 12 etc.. its mainly just luck tho tbh.. its a numbers game, so stock up on the equipment you wish to meld and same with the materia you aim to meld