I would just ignore it. I find it very hard to believe it would be worth using high and low rates for such a tiny difference. Just use the "high rate" value, and learn to be gentle on the sticks.

I have a Hitec Flash 5x, and unless I'm mistaken, it doesn't have separate EPA for up and down. I think it's more likely that the geometry of the plane (the control horns and the servo arms) results in uneven travel, not that you have to set it. Set the up amount correctly, and I bet the down will be pretty close to what they say.

Running out of up elevator is much more of a problem than running out of down, because you need the up elevator to land the plane.