I wouldn’t even bother doing that.
First, we are agreed that the money doesn’t count as income, like building society interest doesn’t count as income.
Instead, the full amount of rent income counts as capital the instant it is received, like building society interest, but only for as long as you still have the money, also like building society interest. I don’t think it is necessary to work out how much rent “profit” has been made: if they’ve still got the money it will be visible in their declared bank accounts etc. If they haven’t, it won’t.
One obvious reason why they might not still have a large portion of the rent income is that they have had to use it for outgoings like mortgage payments. But those details don’t matter – any remaining saved capital is just part of their generic stash, which will be apparent from the bank statement/pass book or whatever.