1.) You will find that very few, if any, manufactuers publish a gradability % for low lift walkie, order selector or narrow aisle equipment as the intended application for these units in on a flat level operating surface found in warehousing oprations.

2.) the ANSI/ASME standards for parking brake holding capability should not be interrputed as the capability of the unit to climb a grade. For example, a stand-up rider, reach truck (North American style) has about 1.25 to 1.5" under clearance at the outriggers. It would not be feasible for such unit to negoitate the break over point on a 15% grade. Further, it is highly unlikely a highlift order selector (picker) can negoitate a 5% grade without getting hung up on the frame side skid plates nor would teh operator want to be at 300" in the air on a 5% slope w/o a parachute or bungy cord.

3.) To operate a MPB 040 on a 10% ramp with a load will significantly shorten the run time on a fully charged battery pack to vs an expected 3-4 hour operating time on normal flat surfaces. Also to ooerate this unit or similar units on a ramp will shorten the life of the typically small drive motors found in them.
You need to confirm with your dealer their recommendation based on your operating duty cycle - the machine maybe able to negotiate that ramp if you only do it once a day. And get it in writing from them and /or Yale.