The slope that drives the water and controls the tile capacity is actually the Hydraulic Grade Line (HGL). Simplified, it is basically the slope of the water table in the soil (if you drilled 2" PVC monitoring wells along your tile main).

People obsess about being perfectly on-grade, and it is a craftsman-like job and helps the tile keep from 'silting up' in low pockets over time, but taking your minimum grade (especially over a short distance) and looking up the capacity on your slope/capacity charts is an understatement of the actual capacity of that tile.


Edited to add:  The assumption for tile is that HGL approximates tile slope over long, relatively flat distances.  And that is pretty reasonable, especially when tile is installed at a generally constant depth to the ground surface. But over short distances and sudden grade changes, HGL does not follow that tile slope.

If you were going to guess-timate tile capacity for this main, I would draw a 'best-fit line' along your tile profile, and use that slope to plan the rest of your system.