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.
I was shown a way to check initial maai that seems to work pretty well (most of the time... nothing is absolute).
While facing uke and looking at his chin/nose area you should just be able to see his feet with your peripheral vision. If you can't see his feet, you're too close. If you see a lot of floor in front of his feet, you're too far away. After a while the correct distance starts to happen more automatically.