As Jim, a boot fitter from Starthaus, told someone else who also weighs 115kg:
for your size 60 or 80 will be the most effective flex for relaxing at the bar and enjoying 8 or 12 pints.
at 250 lbs, even if you do not ski very hard, a boot with a flex rating below 100 will not be of much value for your skiing.
for a female skier skier half your weight skiing on blues or greens, an 80 flex boot would be appropriate.
Go to the "Ask the Boot Fitters" forum, read the wikis about fitting and check the list for a fitter near you. If there isn't one listed near you, tell us where you live and probably someone can make a recommendation. DO NOT buy boots on the internet. Street shoe size means practically nothing in terms of ski boot size.