Develop a .XLS Macro

I have a table that I need to fill in. It is a table that shows businesses what the actual percentage discount they are giving up after a deduction for the people who pay for gift certificate never redeem the gift certificate and the business got free money. Example: Math Walk Through: Business sells 200 certificates with redemption value of $100 for $82 per certificate with 20% non-redemption's. Total Sales $200 X $82= $16,400 instead of the normal $200 X $100 =$20,000. So total discount given up is $3,600 (3,600= $20,000 -$16,400). Or simply ($3,600 = 200 sales X $18 per sale discounted) If 20% of the 200 certificates did not redeem the business freely received $82 from 40 people (40 = 20% of 200 sold). So the total free dollars received is $3280 ($3280 = 40 non-redemption's X $82 paid). So the business total discount was $3,600 but the free money received is $3,280 and therefore the net total discount given up is: $320 Now divide $320 net discount over the 160 people that did redeem and that means the effective discount percentage given per customer redeemed is actually only 2% ($320 divided by 160 redemptions = $2.00 per sale.) This would be like selling each $100 certificate for $98. I need to complete the uploaded table for various percentages of non redemptions and various levels of initial percentages offered. If you can create a .xls macro that would allow me to plug in the X and Y coordinates and find out the answer that would be great. I need to show in the table 3 digits max for the answer. In the example that I did it worked out to 2% exactly but there will be answers that may be 2.54% So only show three total digits either rounding up when above 5. So 3.656% becomes 3.66 in the table.