Pitching onto yeast cake for smaller batch size

I've never reused a yeast cake before. My plan is to make two different recipes of the same style back-to-back so this seems like the perfect time to try reusing a cake. I'm assuming I rack the first beer into the secondary and can then just rack the fresh wort of the 2nd beer into the 1st carboy. But I wasn't planning on making two 5 gallon batches. I wanted to make the 2nd batch only 3 gallons in size. Do I need to remove any of the slurry from the carboy I used to ferment the 1st beer in or will it not matter?

You'll probably be better served to scoop out a good half to 2/3 of the cake; when I re-use yeast from batch to batch (and when I don't plan on washing it), I usually only leave half of it in for the second batch, and that's with dual 5-gallon brews.