severe ankle sprain impairment rating

I had a grade 3 sprain in my left ankle that was not repaired surgically and after a year the work comp company wants to do an impairment rating it was injured while working as a volunteer fire fighter. What is a fair rating i still have pain somedays to the point of a severe limp and after this time I dont expect improvement it has impacted my life and I fear permanantly so not to sound greedy but if they are gonna try and "buy me off" want to know what to expect is fair and if i accept are they gonna write me off or if the ankle does need surgery are they still accountable.