Attorney Fees -
Yep, the fee by law is capped at $5,300. The only other fees they can charge are the "extra" out of pocket expenses. This was disclosed to me up front by my attorney. I received a bill for $90 from them with a copy of the documentation for those fees. If they just send you an additional "list" bill, request documentation of those charges.
Back Pay -
If you happen to have a copy of the annual social security statement they send around your birthday, there is a section that states.......if you were to become disabled today, with a $$ amount. That is a starting point for your back pay. Then they calculate the COLA each year from that. My monthly amount was within $20 of that figure so I was able to reasonably "guesstimate" my back pay before I received my award letter that included the back pay amount.
I was approved in October but haven't received my back pay yet. The reason I haven't is because an SSI review had to be completed and recently was. They are now stating I will have it within the next 4 weeks because the review is completed
Congrats on your favorable decision!