After some testing:
2|5|10|17|26|? Those are thresholds for the next promotion (it doesn't take an Einstein to guess which Promotion threshold is next).
A general has a total of 20 Experience Points to give (a native village offers 7XP sometimes), which are distributed equally among troops in the same tile. The most effective thing would be to lead 10 troops, which get 1 promotion each. *
Four units get 2 promotions each. **
Three units get 7XP+7XP+6XP and 2 promotions each.**
Two units can each get 3 promotions (at XP 2-5-10) ***
One unit can be promoted directly to the 4th rank, with 3 XP overshoot. ****
Free promotions don't affect the threshold, each units will be promoted when it gets it's first 2 points experience, even if it is already a Grenadier I (from the French leader) or if it's a veteran soldier.
If you choose right to bear arms (+1 strengh for citizens), and have the FF with +50% to native strength, then promote 4 natives to minuteman II.
What you get is four 'Chingachgook' units with
3 Str
+45% settlement defense
+50% native strenght
+X% rebel sentiment.

Bring on the Artillery.