I went through this process extensively when I got to ZJX. The basics are that you are eligible to be paid for your time in OKC as that is your entrance to the FAA, and you’re allowed OJT pay as such. However, you are not able to apply for this until you have graduated from the academy and arrived at your first location.
If you already have everything in the system and have used even a little bit of time, it should be very easy to set your Application date to the first date you show at the academy, submit and get back paid for the academy time as well as get your monthly payments from there on out. It will start at 100% of the BAH rate for your area of an E-5 if you are on Post 9/11 bill. Montgomery I’m not sure as there aren’t many of those left. This 100% pay then decreases every 6 months you remain in training by 10%. So 1-6 = 100%, 7-12 = 90%, so on.
If you haven’t submitted any paperwork or used your GI Bill before ever, then you must ensure that your date for claiming pay is the first day you started at the academy. I was under the Montgomery GI Bill, and in the process of switching to the Post 9/11 while applying for this my paperwork got royally messed up and led to me being extremely delayed in payments.
If you have any more questions about it feel free to ask.