I don't personally use garlic powder or onion powder in my pizza doughs so I can only tell you what I have read on the subject from people who know the effects of using those powders in pizza doughs. So, on that basis, you might take a look at a Tom Lehmann Think Tank post at http://thinktank.pmq.com/viewtopic.php?f=6&t=7797&p=53267&hilit=garlic+powder+lehmann#p53267
where Tom suggests that garlic powder be used at 0.15% of the total flour weight. However, I do not believe that 0.15% is the upper limit since, in the first Q&A at http://www.pmq.com/mag/20070607/lehmann.php,
Tom specifies a value of 0.25%. In your case, with a total flour weight (All Trumps and semolina) of 1715.93 grams, 0.15% comes to 2.56 grams and 0.25% comes to 4.29 grams. I further wrote on this subject not too long ago at Reply 2 at http://www.pizzamaking.com/forum/index.php/topic,9320.msg80647/topicseen.html#msg80647.
Whatever you do with the garlic and onion powders, you don't want to go overboard, as I discussed in a post at Reply 8 at http://www.pizzamaking.com/forum/index.php/topic,6680.msg57325/topicseen.html#msg57325.
In that case, as noted at Reply 11 in the same thread, the poster noted the deleterious effects of using too much onion powder in the dough.
abilak is bound to have considerable experience with levels of garlic powder and onion powder to use. However, in a dough formulation that abilak used earlier this year, and which I converted to a baker's percent format at Reply 5 at http://www.pizzamaking.com/forum/index.php/topic,7689.msg66071.html#msg66071,
by my calculation abilak used 0.80% each of garlic powder and onion powder, apparently will no ill effect. So, it looks like you have a pretty wide range of values to play around with.
Good luck and let us know how your use of either or both of the powders works out.
EDIT (4/18/13): For the Wayback Machine link to the dead link referenced above, see http://web.archive.org/web/20080321081127/http://www.pmq.com/mag/20070607/lehmann.php