measure your panel, both directions ~ you may have to trim in a smidge to get a "normal number" + 1/2" seam allowance ~ make it divisible by 2 or 3 or 6 or whatever size finished blocks you want... then do the math for how many blocks you need.
Before you cut outer blocks, tho, add your 1/2" to get your 1/4" seam allowance on all sides!!!
Sometimes running the math backwards to check your totals before cutting is helpful, too. Then if you want quick finish, you can strip-cut your blocks (in groups of 3 strips or whatever)