Convert the length of a side into inches and multiply it anywhere from 4%-6%. This gives you the apex of the curve. For my particular tarp however, at 5% for the longest side, it takes off 6.3" and 4% took off 5" which is not that much of a difference. For the shorter lenght of the tarp, the reduction is 4.2" That's a scary cut! Anything less than 4% supposedly is not going to be that effective. I'll have to be very careful with this.