Keep The PR of the differeent levels of the stack very close to eachother, have the ships that take the most damage at the front and the ships that give the most damage after that. And just experiment with different ships.
And if you can use the gaurdian G. Livid(R) and G.LIctor ships once researched as they are very good attack/defence with very little PR.
Good luck! Any more questions PM me.