When i use the free action to Treat, do I measure from my unit, or from the unit leader?

"When measuring range betwen units for any game effect other than an attack, range is measured from the closest mini in one unit to the closest mini in the other unit. " rr, p. 49.

So it is measured from your closest mini of the unit with the "Treat" keyword to the closest mini of the unit you want to target.