Ridgerunner, I have a long interest in motorsport and looked into fire resistant clothing many years ago when we started to be issued with Nomex flying suits. You will get a lot of information from motorsport.
20 years ago the info was something like: Single layer nomex gives about 15-20 seconds "full flame protection", that was for normal petrol, AVTUR???
It chars/degrades and after that time starts to lose its integrity. Add suitable underwear and the time rises to about 50 seconds (Nomex best, cotton/inert OK). You could however "boil in the bag" with just the suit, ie you dont catch fire but the heat will boil sweat etc!!
The important thing is that your clothing would not actually catch fire.
Sadly one of our guys only had synthetic underpants on underneath, they melted under the suit and stuck to his body. (Chinook, Hannover Airshow, 80's). Also the suit needs to be sewn with Nomex thread and the early flying suits were not, it was very easy to demonstrate with a lighter and length of thread!!
With a flash fire the important thing is to be covered so the boots/underwear/helmet/gloves/vizor do a pretty good job.
Its an interesting subject!
lsh