I may have mentioned this before, but there's an error in the Open to Buy table on page 143 of Friendly Local Game Store. The calculations are wonky starting on the second line. Here's what it should look like, based on the data in that table:
The important calculation on each day is the Balance column.
2-27, is $1,000 (starting balance), plus COGS on 2/27, minus Purchases on 2/27.
2-28 is the Balance from 2/27, plus COGS on 2/28, minus Purchases on 2/28.
2-29 and on, just copy the formula from the Balance on 2/28.