Regarding making the option configurable, the intention is to let the users experiment with different values to suit their needs. Also, if percentage is not the right way to represent it, then would a float factor < 1 make sense?

Regarding the misleading name :-) I can think of another name something like fragmentationFactor. Please let me know if that sounds ok or if something better is on your mind.

Regarding the value of the factor itself, I did not quite clearly get the comment. I guess you meant:

1. factor value=75% then split the new entry if it is <75% of the entry's space.
2. factor value=25% then split the new entry if it is <25% of the entry's space.

and not the other way round (which is implemented as of now) - it's kind of reverse as of now.