Refactor implementation of -Fd flag to make it clear that it's not the F factor which is scaled

In #21483 (closed) we had a discussion about the -Fd flag and whilst concluding that the behaviour was at least not very wrong, we did agree that the flag is not really scaling the -F factor at all.

The -F flag sets the multiplier for how big the heap must get in order to trigger a major collection. By default F = 2 so the heap must grown to 2x the last amount of live bytes before a major collection is triggered.

The -F flag is used in the calculation about how much memory to retain but really we are not scaling the -F value because -Fd doesn't affect how quickly we will decide to do a major collection in the future.

Therefore the implementation could be refactored a little to make it clear that we are not scaling F, even if the ship has sailed for the variable name.