This doesn't answer your question, but if you're trying to use firehose for high-pressure water feed (off the discharge of a pump or something), may I suggest not using it: I just tried that on a valve test rig I'm designing/building. The layflat was rated to 300 psi, and I was discharging at only 200, but man did that stuff stretch (like, 3/8" growth on a 2-1/2" line), and flex, and make it hell for me to seal the fittings at the ends of the hose. I think part of the fact that it's so much cheaper than proper mandrel-built rubber hose is that it's not meant to be used every day
'Lay flat' hose is usually used as a discharge line from a pump. Not a supply line except for fire-fighting equipment. Talk to local fire Dept. about the losses. They may be able to give you pressure head at the Pumper truck and a good guess on what is coming out of a 100 ft. hose without a nozzle.
