the strength of a unit, excluding abilities and speed, is approximately equal to:
((average damage*health)/2)*(1+(attack+defense)/50).
The results calculated using this formula are roughly equal to the results from Zingel's formula, although there are some changes.
Applying abilities will yield the actual result.
We can now proceed to discuss the ability coefficients.
Ranged attack adds approximately 20-25% to a unit's strength, flight - from 10%, double strike increases damage by about 50%, own-turn strike - it is unclear how to evaluate this for now, extended attack increases damage... let's say by 20%, unlikely more. Mass attack - increases damage by 100% (possibly). Vampirism and dendroidism must be evaluated separately, as well as unicorn abilities, manticore abilities, medusa abilities, and other tricks like resurrection.
Magic immunity also provides some bonuses.
Speed apparently gives a boost of 5% per unit.