Percentage of month passed...

I have found that I need to keep a daily eye on the amount of data that my family uses on our cell phone plan or else pay the outrageous fees for going over. I'm trying to come up with a spreadsheet that can show how much of the month has passed (as a percentage) and compare that to the amount of data that we have used so far. The thing is, our plan goes from the 29th of one month to the 28th of the next month. I found a formula to calculate the percentage for a standard month (1st-30th or 31st), but can't seem to find a way to modify it for my date range needs.

So, when I see that we have used 4% of our data and it is the 18th of Feb, I want a quick visual of what percentage of the plan's month (any month) has gone by and see where we stand.

Thoughts?

Exactly what I needed Skywriter. Thanks!