We've got discounts for annual and semiannual payments for some of our
accounts also, but are not running into the problem you are. Our numbers
are coming out perfect (Emerald 2.1.11 was off by about 3 cents).
Annual discount: 20.8417% off
semi-annual discount: 16.6667% off
The annual discount is based on paying for 9.5 months and getting 2.5 free.
The percentage was modified very slightly from 20.8333 to make the totals a
bit more "even" after calculating.
The semi-annual discount is based on paying for 5 months getting 1 free. No
modifications were made to this.
24.95/month paying annually winds up being 237.00
19.95/month paying annually winds up being 189.50
24.95/month paying semi-annually winds up being 124.75
19.95/month paying semi-annually winds up being 99.75