Not to nit-pick (because I do like the idea), but this gives you the mean, which can be skewed by erratic numbers on 1-2 days (let’s say Feedburner reports a 0).

For example:
Day 1: 500
Day 2: 500
Day 3: 0
Day 4: 500
Day 5: 500
Day 6: 0
Day 7: 500

The average is going to get reported as 357.

If, however, you use the median (create an array with the 7 values and select the 4th one), you’ll get an average of 500 – which seems to be the correct value. As a bonus, you’ll always get an integer.