I once talked about Open to Buy at a trade show and a guy came up afterwards with a bunch of people surrounding me, and told me my example numbers were off. He was roundly criticized for bothering me with minor discrepancies from an example, but hey, examples should be correct. But that also meant I've been reluctant to start throwing around C2s-E2s because they would inevitably be wrong somewhere. If you do find a discrepancy, please let me know.
This example assumes you're starting with no money in your purchasing budget. On day one you have only your cost of goods to start with and your purchases to subtract from. Then we just add that balance going forward and voila, you've got a spreadsheet you'll live with for the rest of your professional life. Create a shortcut. Back that puppy up.
Here's a link to this example file.
You can copy and insert lines like this forever, especially once I discovered that +1 trick with the date. Mine is nearly 5,000 lines long and has a ton of columns, along with many financial charts that use this data (which I prefer to Quickbooks).