Retraining units

Lets say i went in battle with 3 spear militias, 3 lower militias and 3 pavic crosbowman (venice).
Everyone suffered some light causalties so i have 9 wounded units. I retrurn to city (with 3 recruitment slots).
If i simly strated retraining, it would took 3 turns (9 units).

But if i merge wounded units, i will get 2 healthy and one almost depleated for each unit type. So i have 3 almost depleated units and can retrain them in one turn.
Maybe its common knowladge, but i didn't found it anywhere and it realy speeds up recovary after battle.

Btw. if i retrain unit, does newcomers starts with zero experience (so exp. will get averaged and fall), or they gain same rank as retrained unit?

Its common knowledge and is widely used, mainly because retrained units are added at the same experience level of the retrained unit.
I.e. retraining a 9 valor unit will result in all new recruits to this unit being 9 valor as well.

Also, I will oftern save a game and then experiment with merging units to get the best experience out of them. I doubt the AI does it as well because if you auto merge everyone you oftern lose a lot of small high level experienced units in the shuffle. Having many gold and silver chevroned units really helps when fighting the Mongols and Timmurids. It's another micromanaging time killer.

you reload in order to optimise merging and then moan about micromanagement?
lol!