I don’t think that you need a mathematical function.
One attribute of your leaf collection would be “% Hydration” and you would enter the calculated result there.
To make this easier, I would suggest a free text entry per sample with a reference (sample #1, sample #2, etc) so that you know which calculation belongs to which leaf.
Obviously you need lots of leaves with different starting hydrations.
You also need to be sure that your dehydration is only removing water.
regards, Roger