I charge a daily fee as well but I get paid monthly. So when I do up my invoice, I count the number of days in the month and send the invoice for that amount. Otherwise, you end up coming up short. I had also made the mistake of putting X mount per month in the contract, but my client (who I now really like!!) was like "so we'll count the days each month?" And I thought, Of course!! That's how it should be. Hope that helps.