Substat question!

[Maero]

Having been a blm a main and now more or less Red Mage i can tell you this expansion determination is the least valuable stat.
For Red Mage i prefer Crit/DH and only time i do meld determination is if all other stats are capped on the gear.
I know not the impact of DH on Dragoon but this expansion seems Crit is one if not the best stat for most jobs.
The Ariyala website can help alot with planning your gear and melds and trying different combos.
http://ffxiv.ariyala.com/

[Cetonis]

Just as a small addendum, Det is about 94% as effective as DH, so you would pretty much never give up mainstat or total substat points or anything just to swap Det for DH.

[Kaozurei]

Thank you everyone for the replies! Substats are still pretty confusing, but at least I more or less have a better idea about what I'm doing.