Honestly, I don't think I'd make it all that complicated. Leave it with k*Mark/Cap formula and have at it. Starships will "waste" some boost (since their base DPS is half normal) and Turrets will be a big gain (since they have relatively x3 DPS), but in the grand scheme of things, I wouldn't be overly concerned. We've already got Military CS boosting Turrets, so it isn't like attack boosts are impossible to get on every Turret in a system. If you want to micro attack boost for turrets that have damage multipliers against the current wave, good for you. I don't think its any more powerful than the abuses possible with attack boosting right now. It's just more clear and consistant.
The only issues I have with using the strength metric are:
1) It isn't an obvious one to the player. We don't know how much strength every unit is worth (and I don't think we need our interface clutter with that information)
2) If strength takes into account health (so more health means more strength) then that would suggest it wouldn't be any more accurate than my suggestion
3) If strength takes into account actual DPS then it really becomes impossible for the player to evaluate how much can be boosted because of the wide differences in per-ship dps