Implementing a mileage cap for expense claims

I would like to somehow establish an alert or per day column when using bills for mileage claims. In the bill's 'Item' section, I make 3 line entries for 3 separate days' worth of mileage at various miles per day and this is ok, but if my organisation has a cap of 16 miles per day that can be claimed, how can I set up an alert to highlight this if more miles are entered that should be? Can I create some sort of rule of restriction or notification pop-up?

Perhaps there are other suggestions for how I could manage this?

Any advice will be appreciated.