seank

I consider ma ai to be the range where with one step toward me an uke can actually make contact with me, and where with one step I can change that distance as nage.

In reality though, I would suggest ma ai is almost a state of mind, wherein you know that you are either beyond reach or in imminent danger of being hit. It is fluid and depends on the movement, position of uke and nage and also whether uke or nage has a weapon in their hand.

Something I have found useful for getting a feel for ma ai (unarmed) is the average of the height of yourself and uke as a horizontal distance between you. For example an uke of 180cms and and nage of 185cms would be a distance of 182.5cms. Mind you though, its not practical to consider this in a fighting context, but can be useful in basic training.

As related from the article above, it does depend on speed and timing as well.