I usually go down to 80% ground combat and make up for it with extra troops. As a funny aside, what do people commonly use for troop transports? In a game I am in now I am using anything available in my fleet that has cargo storage. Since in simultaneous play, your transport drops all of its troops you cannot go on a rampage and take a bunch of planets in the same turn unless you have multiple troop transports. I found myself in lightly defended enemy territory with a big fleet that included 1 troop transport, 2 mine layers and several colony ships. I took some of the troops out of my transport and put them in the mine layers and colony ships! Then I just plot a course for as many planets as I can move to in a single turn. I laughed when I watched the combat replay and my little colony ship moved up and dropped its 2! troops and conquered the planet.
Although I had thought the local militia would have been enough to put down 2 troops