rebill only part of a mileage expense to a project

When creating a mileage expense and selecting the "Rebill to Project" option, as well as specifying the rebill rate, could there be an option to select how many miles to rebill as well please (which would default to full amount of miles being claimed).

I have a Client who a fellow Director does sporadic work for at a variety of locations, and we've agreed an mileage expense rate of so many pence per mile, less the first 30 miles.

When it comes to invoicing this Client, I have to manually calculate the new mileage amount and modify the rebilling entry on the invoice, which is a bit of a pain in the rear end.