So ..... 25 inch elevation change per 100 feet? Technically drain line grade and size should be determined on potential maximum flow and required flow velocity. That said, in this case the OP can just work with percentages to make it simpler. Code dictates grade requirements are 5% minimum slope from foundation (6" over 10 feet) and 12" + 2% lot grade from top of foundation to curb. That said, your cities building department will tell you exactly what is required and/or recommended. Probably the safe bet is set the drain line grade to 1-2% minimum slope from your collection basis to discharge point. .