We haven't done this yet, but I like this idea.
As for the amount of the tip...yes, I pretty much know what we will order at each restaurant. (yes, we are very predictable) I will look up the prices before we leave and calculate accordingly. Also, DH always has money in his pockets, so the tip can be adjusted.
I think the main point of the tip envelopes for the wait staff is to have the majority of the tip already ready and easily accessible.