ef00 is the correct code for UEFI boot partition.
The sidesize does not have to be 300MB, I would say at least 100MB. I generally use 512MB for no real reason other than it is large enough that I don't need to care and I like the number. But itIt only has to be as large and the kernels and confisconfigs you store in it (with some room to spare). Mine currently uses 36MB on one of computers and 26MB on another so plenty of space.