When i make my ships i always (when i make two that are about the same power) look at the different attacks and HP...and then i determine depending on what the ABS is that...
lets say my ship has 1.000.000.000 HP...and my shields are 65% then i round that to about 50% and determine i can suffer more than 2.000.000.000 Dammage...then when i look at the attack (750.000.000) about i realize that any ship around my power won't be able to destroy it (besides bording) since there is no way they have over 2.000.000.000 attack power (after ACU kicks in) if it is the same cost as my ship