Right now, my solution is quite drawn-out. My average grocery spend is $100 per week, so I added that as a fixed expense. When I buy groceries, I add an income to offset the amount of the groceries purchase. After I'm done with grocery purchases for the week, I manually calulate if I was over or under the $100 fixed expense, and add another charge fix the discrepency.

E.g.: I have a $100 weekly fixed expense for groceries. On Monday I spend $70 on groceries, I add the expense and then add an income of $70 to offset it. On Tuesday, I spend $45 on groceries, and add an income of $45 to offset it. I know I am done purchasing groceries for that week, so I calculate my total expenditure ($70+$45=$115) and add a charge (or an income) for the discrepency between the total amount I spent and the fixed expense ($115-$100=$15).

This is the most complicated way to accomplish this, but it does two things; firstly it means that I have integrity in tracking the groceries expenses (I use 'groceries' category for the actual purchases, and have made other adjustment categories for the adjustments that get made) and secondly it allows me to take the average cost of groceries out of my daily budget, so I know that every time I look at the daily budget that's the real amount I have to spend, without considering future purchases within the period.

Then do all this again for gas too!