Perform an iodometric titration on the H2O2, UTFSE for the procedure regarding that, I think I might have even been the one to post it, if not I am sure Rhodi has it on his site, or it can be found somehere on the hive if not pm me.
Formic Acid 80%+ in my experience has always been supplied in HDPE with vented caps, and not glass. Again UTFSE for Formic Acid, Glass, and it burning someones house/barn down.
I with out a doubt would not perform the reaction without at least calculating how much H2O2 by titration. I once had a failure due to very old H2O2. Even though it may proclaim that it is a stabilized solution, the stability of the solution is not expected to be stable over the course of three years. Especially if it has not been stored at 2-8C
regards,
m_e