Li Shenron
Legend
I didn't say it was your fault.
I'm saying it's not a good idea to argue the feat might be reasonable because its limitation. That limitation is one you don't want to enforce.
I like that limitation instead. I like the idea that I have to choose after seeing the dice but before knowing the outcome. In most cases it means I would use the feat when rolling low, but not when rolling average (although there are always cases when you can derive the roll you need based on previous rolls). The higher the original roll, the riskier the gamble. It kind of vibes well with me for a feat that represents being Lucky but essentially works by manipulating chance. Sometimes chance strikes back.
There are other abilities in 5e which allow you to choose after the outcome e.g. maneuvers that you use when you hit rather than when you attack but before you hit. But there are also many which you have to choose before you even know they will be applicable. Lucky is in the middle.
Overall the feat is still strong, but the limitation keeps it balanced. But obviously if you don't like the main benefit and neither the limitation, just ban the feat and write a completely new feat.