Sensei Strange's ma-ai question from a day or two ago...
Go back, for a moment, to swordwork. When you measure ma-ai in swordwork, you don't measure by touching the tips together - you cross swords by about a fist-breadth or 3 inches or so. This goes back (I think) to the discussion in The Sword and the Mind about 'sneaking three inches'. Uke wants to find the distance to attack from, where he can put his sword about 3 inches into you with one efficient lunge. Same thing applies to arms as well as swords. Uke does not want to lunge forward just enough to barely touch your body with his fingertips - he wants to lunge far enough to put his palm on you, representing a fist penetrating 2-3 inches.
...the other day you mentioned finger tip to finger tip is incorrect. Why? I practice finger tip to finger tip, just because I feel handblade to handblade is actually too close. I feel Maai has already been broken...
Also consider, tori does not want to begin his reaction when uke is still outside of ma-ai because that makes tori's motion easier for uke to read and follow. Tori wants to define ma-ai as the distance where uke can first begin affecting him. At finger-tip to finger-tip, uke can, in no way affect tori. He is just too far away. But palm-to-palm represents the distance where uke can casually lean in and grab your arm if it were extended.
There's also the subjective feel of the different distances. Measure fingertip-to-fingertip and put your hands down. Looking at uke at this distance tori is alert, but not especially anxious. Uke is also not particularly eager to attack because he can intuitively feel that the distance is too great. But let uke shift forward a couple of inches and all of a sudden tori can hardly contain himself because he knows he is right on the edge of his ability to evade if uke attacks. That's ma-ai - you want uke's presence at ma-ai distance to make you so anxious and alert that you begin your evasion at the first sign of motion from uke.
So, ma-ai represents the distance at which uke first becomes an immediate danger to tori - but also a great enough distance that tori can still manage an evasion if uke attacks with maximum efficiency. That distance is generally closer to palm-to-palm than it is fingertip-to-fingertip.