Doh! Let me try again. Even someone as brilliant as myself might still be uncertain after reading my last shot

Think of there being two types of square metres for which you will incur tier charges.
Tier applies to area in excess over the 512 metres 'free of tier' for a Premium account.
The first and most obvious type of square metres are the land that you own in your name.
The second type is the square metres of tier that you donate to groups.
Keep a running total of the two types.
The tier that you will be charged at the end of a month is whatever maximum instantaneous combined total the two get to during the month.
Even if you are at that maximum for a few seconds, that's the area that will determine the tier level.
Hard case example:
You own 512 metres in your own name
You donate 1025 of tier to a group. 30 seconds later you have a moment of clarity and remove the donation.
At the end of the month, you'll be hit with the 2048 tier level.
Your 512 will have been free of tier.
Your donation of 1025 was 1 over the 1024 level, so it put you in the next level up (204

.
Whether or not the group owned any land is immaterial.
A separate question - but what the hell, it sort of belongs here:-
What happens to a group if a member withdraws a donation, and the group then does not have enough tier to cover the land owned?
Does LL send a message to the groups owners? Someone has to pay the tier.
Does LL just disband the group and take all the land? - as apparently it does if the number of members falls below 2