Help with materia melding.. thx

hi,
i follow this guide to do materia melding
http://eorzeareborn.com/a-materia-overview/
from there i quote:
" there is a relationship to a cap on stats an item can have to its item level. So if you have a +3 acc and a +3 crit materia with a piece of gear that has +acc already you might want to put the +3 crit on it vice the +3 acc. Since you might only get +1 or 2 out of the +3 acc materia, but if you had a similiar piece of gear with a higher item level you might be able to get the full +3 acc."

and this guide: http://www.ffxivinfo.com/materia/guide.php

i think what keep happening is related to that, but i just cant figure out what the "cap" is

i did a materia with spell speed 2+, the gear already had speed speel 2+ on it.. so when i attach it it yield zero in the materia area; but then i had one with 3+ and add an +1 materia and under materia has plus 1 which i assume adds to the one already in the gear... in both occassion the gear and the materia where same lvl.

i also understand that the tier of the materia has to go with the lvl of the gear (which is not always the same as the lvl u can equip).. so far all my stuff are baby lvl,materia tier 1 and gear lvl 14-21

i dont understand how to to check this cap..or based on what is determine...
or why i sometimes waste the materia i melded
any ideas? cant find any answer in google...
thx :)