Sorry about your injury. Time to heal is very important. I skied injured for the past 2 years. Tried to "ski through the injury" the 1st year. Bad idea.
You asked how to know if flex is right for you. Here are my thoughts. My perfect boot is the Dalbello Krya 85 flex, or so I thought. When I first bought the boot I was concerned they were too intermediate for me. However, they were perfect, easy to get into, the right amount of performance, comfortable for my small 22.5, high instep foot. However, something was missing this year. Not sure if it was due to more strength in my injured leg, more strength overall, going from a 96 mm ski to a 82 mm ski or the fact that they were just to packed out or all of the above.
Last week I went to my boot fitter. 1st he checked the flexibility in my ankles by asking me to squat as low as I could go. No problem in that department. My personal trainer helped me perfect my squat. I can squat to the floor. I put the new Krya on my left foot then compared other boots to it. On my right foot we tried a 22.5 Technica Mach 1, LV 85 flex. Wow, a much better fit. Compared to the Krya it filled in all the spaces in my foot especially around the top of the foot, ankle and shin. Then I tried on the Atomic Hawx Ultra 80. Ouch, it was too tight even without buckling the boot. The fitter stopped there. Said the Technica boot was the best fit for me. He never asked me to test the flex, just move around like I was skiing.
I only got to ski the Technicas once this week but what an amazing difference. I was on my 82 mm ski, which is light, lively and turns quickly. First I noticed the "uprightness" of the boot. My old boots allowed more fore/aft positions. My boot fitter says boot uprightness is due to the way modern skis need to be skied. After a few runs I realized how responsive the boots are. The only way I can describe it is my turns on the Dalbello's were slower, relaxed while the Technicas wanted me to get on them and go. They really bought out the liveliness of my skis. Can't wait to try them on my wider skis.
So my assessment is flex is only one part of the equation. I'm 5'1", 100lbs so an 85 flex works pretty well for me. Maybe a 95 flex would work better in the Krya's. I think it all depends how a boot fits, what it is designed to do and a person's skiing style.