Q: compare boots flex index from different brands ???

Hello,

I am looking for stiff boots and confused with different
"flex index": Solomon count 55, 65, 80... Atomic 7,9,...
How one can get flex of Technica vs Lange vs Nordica?
They do not mark it on boots....

My reference points: fast Volkl-p40
Atomic CX 9.10 -- soft on high speed
Solomon performa flex 55 -- better, but not mine
Solomon XWave flex 80 -- feels great 5 min in ski shop

Thanks in advance for any general or specific advice and internet pointers.

Alex V.