Thanks for the added info!
Hmm. Thinking this through a bit, what about a "sacrifice X points of ships to get away" option, sort of like when you're picking ships for pursuit? That would have the advantage of letting you know exactly what you need to drop to be able to get away, and wouldn't belabor the process. Made a note.
That would be ok. Honestly, anywhere you put this information in the game, I think would be fine. I wouldn't mind if the information was available in the fleet screen somewhere or before an engagement like you suggested, but I'd rather know it before you get in a "guaranteed ship loss" situation by misplaying in the sector map. For example, some kind of simple summary of your fleet that would be written in the fleet screen, or on the screen where you choose which officer pilots what. Like small fast fleets would have "A small and very fast fleet, can escape any unwanted fights". Than a larger fleet would have "An agile fleet, however not small enough to escape every engagement" and an even larger would have "A formidable fleet. It's sheer size prevents disengaging properly."
However, I think it is crucial that you can get punished if you misplay in the sector map, and that you can and and will get punished for getting caught by a fast fleet while sporting huge number of cargo ships and tankers, or if you get caught by hunter fleets. Taking the possibility of save scumming into the decision process of resolving this, and just throwing the punishment for misplaying away just because the majority of people will save scum would be a mistake in my opinion.
If you think about the rear guard options that people suggest (I think it is definitely an idea worth exploring), just make sure to make it reeaally hard. Like maybe the ships you choose to distract the opposing fleet start the fight surrounded or something.