Re: Good distance ?
I'm with Dan on this. Maai, or mai ai, depending on teacher's pronounciation - seems funny - has always been treated as a noun in my classes, both as student and teacher, not as a verb. We (Tomiki school) practice closing to learn to recognize maai, to maintain it, to break it, change it based on who can do what to whom, etc.
On the original post's question, basically asking where to put weight, on which foot or percentage divided between each, there are the following thoughts.
... if your weight is primarily (say more than 65%) on the back leg you can really only go forward without doing something strange.
... if your weight is primarily on the front leg you can really only go backward easily.
... if your weight is evenly divided between both feet it is equally possible to go in either, or any, direction, though you can't get "there" quite as quickly as if you loaded up on one leg (potential energy).
With the three above things sort of out there, then it's a question of "how do you fight?" so to speak. One instructor I have never goes backwards, at least I've never been present when he has. I'm sure he knows how, but he just doesn't do it. He is almost always back foot loaded, for example. Since I don't see so well, I can't do that. Also, all the years of kick-punch with power leg back put me in a comfort zone for slight front-loading. It works well for me. I think you should expirement and find out what works best for you.