land permissions

Hi....looking for wisdom regarding land settings. I have around 8000m.... It's set to my group so that they can build. Ive recently offered a resting spot to a friend, and cannot for the life of me work out how to set it to a certain number of prims.
I divided off around 1500m, and set that to allow objects to be created, figuring that as soon as it hit the smaller parcels prim limit of 375 prims, that would be it "parcel full". But it simply carries on using my OWN allowance. Is there any advice that anyone can offer on a failsafe way to create a sub plot that hits its limt and then stops?....confused.
Thanks in advance.

make the 1500 sqm of land....owned by a different group add friend to group and that should work