In the absence of any provision for the changing of apportionment, being a provision which every leaseholder agreed to at the creation of these arrangements, stop worrying. There is no way that anything could be done without the agreement of every leaseholder. Since any change penalises some, that is unlikely to happen at any time and certainly won't while you hold a lease.
This party will have to demonstrate how this difficulty for her is to be overcome. Ignore her. I wouldn't bother to write anything. Saying "Don't bring it up again" will never stop her and there's no legal remedy to stop her either. If you must, ask her for chapter and verse, in law and supported by evidence, for what she proposes That won't stop her ether, I suspect. I have some experience of this type; for years a Parish Councillor has been claiming that a path over my land is, or ought to be declared, a public right of way. That I have shown and explained to her that it is not and won't be, the other Parish Councillors agree with me, the County Council agree with me, lawyers have advised her, but still she persists. Such people are blinded by what they want.