Pre-Op flexion is a good indicator of post-op flexion, but definitely not a guarantee. Many here have found their TKR didn't miraculously fix their limited bending. It might improve it some, but one has to go in with no great expectations.
Keep in mind though that many here DID gain a lot more flexion than what they started with. It truly does depend on the person and what your knee has been through.
I say this from experience. I thought my prior stiff, scarred knee would be much looser after my TKR last May. So far, at almost 8 months, it's not yet close in terms of bending and gives me lots of trouble with stairs and biking. It was worth it though because the pain and instability are GONE!!I know it will probably loosen up some more, but I'm trying to have zero expectations, just in case. so if this is all I get in the end, I'll be satisfied, even if not "thrilled. "
I think the key is to just not set your expectations too high..... Just go with it, see what happens, and hope for the best. Best of luck to you.