How we can get the percentage between two contract fields after each 20 lines or after the last line

Currently we have a requirement to show a total at last of each page for contract lines. Let say we have 30 contract lines, we are printing 20 lines at first page then remaining 10 lines on next page. so we have to give total of 20 lines on first page and on next page we have to give total of previous pages lines and current page lines. this we are achieving at rtf level using <?add-page-total:InvAmt4?> and <?show-carry-forward:InvAmt4;'PT999G990D00'?> tags.

But there is one percentage field which is percentage between two contract line fields. we have to show this also at page total means after each 20 lines or after last line. I am not able to achieve that logic. If someone have any idea, please share with me.