30x30 land protection implementation in GCAM

[jangoepel]

Dear all,
my task is easily described but harder realized. Any input is welcome.
The task:
restrict land use expansion to a level that conserves 30% of natural terrestrial land cover first for a single GCAM-land region and later globally
The problem:

1. As far as I have found out, the XML input files for scenario manipulation/policy settings requires me to set a limit to all natural land leafs (grassland, shrubland, unmanaged forest) for all individual regions in question in the sense that I apply a tax to all conversions from natural to anthropogenic use that would quantitatively subtract from the 30% conservation requirement. The problem is, is there even a way to set a relative limit (%) or do I have to extract the initial land supply absolute value, and calculate the 30% threshold from the original thousand square kilometer value and then input the 30% value in absolute terms (thsd sqkm)?
2. Are there any such studies (or scenarios) available that would make my work easier? I have searched for quite some time and haven't been lucky so far.
   Again, any input/ideas is appreciated a lot.