Unit Merging
way to determine level of merged unit
first, lets assume that it will always take X experience to reach level N. So a unit's experience total will always determine its level in a universal fashion.
Now then, example is unit A and unit B are merging.
unit A has n1 soldiers and unit B has n2 soldiers.
unit A has Z experience and unit B has Y experience.
(Z(n1) + Y(n2))/(n1 + n2) = experience of the new unit C
to clarify, you multiply unit A's experience by the number of soldiers in unit A, do the same for unit B, add the two together, and divide by total number of soldiers in the merged unit.