You seldom want to use the entire cake.
I will occasionally use a full cake, but that's only when I'm brewing a very, very large beer (RIS or large Burton Ale) but in those cases, I'll use an entire cake from a very small beer (typically a mild or ordinary bitter). In other words, I'm using a very small beer as a 5gal starter for a substantially larger batch.
Back in Ye Olden Days when I was first learning how to reuse yeast, I started trying to narrow the gap between the first and second beers. I did this with great success (although it stressed the hell out of my temp control). Eventually, I got it into my head that there must be "a rule of ten." That is, if you pitch a beer ten points higher, you'll be okay.
This was incorrect. Terribly incorrect, in fact. Yeast are incredibly stupid. They don't know anything about math, much less our societal convention for base ten arithmetic systems. They are quite good at fermenting and when you pitch a 1.060 beer onto a 1.050 cake, they are jaw-droppingly good at fermenting--unfortunately they're quite bad at fermenting beer in such a circumstance. The result was a vile mixture of fruit cocktail and bacon. It was my only dumper, but perhaps these are prized qualities in a NEIPA?
There are a lot of ways to re-pitch yeast, most are unnecessarily complicated. You certainly
can go wrong, but you often won't go wrong by simply scooping out a pint-sized Ball jar worth of trub/yeast and re-pitching it the same day, or following weekend. If you're doing a lager, or a substantially larger ale, use two pint-sized Ball jars.
Also,
this may be helpful in understanding how yeast work.
I hope you found this useful.