I've never used FFG personally, but I've used P substitute successfully which some people on the forum are not fond of using P type at all. It was what was in stock under a time deadline and it sure beat using modern centerfire cartridge smokeless powder of any type for SEDS multistage comp. Since you have an ample supply of it you can ground test frequently and develop a charge load for your application experimentally. I ended up running a charge calculation based on FFFG, it was quite small at 0.1 grams. Experimentally incremented in 0.1 grams until 0.6-0.7 grams of P type got a wadding a wash rag about 15-20ft out of a scrap section of airframe tube 29mm MD airframe same volume as calculated from when a motor was inserted into real airframe etc. Then if you have any spare airframe tubing and wadding test it in the scrap airframe tubing with a wash rag so you don't trash your expensive rocketry gear first... I crisped a cheaper chute first try with nomex protector, it was a top flight chute. You can seal off the end of the airframe tube scrap you want to test with HVAC tape or another method of your choice. We later tried the real airframe with shock cord, wadding, and a chute, after we felt confident in charge amount.
You might want to start with FFFG calculation then try FFG BP in slow increments since grains are coarser. I wouldn't want to risk the real rocket kit or scratch airframe until you've got it tested several times in scrap tubing first on ground. I wouldn't have any doubts of FFG BP because it will be safer than any smokeless powder, but it will need ground testing first.
The student rocketry group I was with we were using 9/32" automobile vac rubber hoses compressed charges and sealing it then igniting with MJG FireWires. We later moved to HVAC tape tightly wrapped to save mass and packing volume. HVAC tape was sticker than duct tape, but wrapping it right took some practice else it would leak and burn rather than pressurize. We tried masking tape first out of curiousness.
If you want to do really high altitude stuff, the Jim Jarvis guy has a real cool T-shaped charge container you can make and it works way better than what I recommend. I'm still new at this too so take what I say with grain of salt. Hopefully I didn't rant or give too much bad advice...