No one can answer your question. OSHA/ANSI says forklift operators travel down a grade with the load up hill. That is the guiding principle.

A couple things you have yet considered in trying resolve your issue and that is the full amount of mast back tilt that is on the forklifts. Back tilt helps "cradle" the load, the more the bette but the manufactures requires certain back tilt based on the mast type & lifting height. Here are some common back tilt angle and the slope equivalent
Back Tilt Degree = Slope %
8 14
6 10
5 8.75
Now these angels are measured with new tires front and back - as tire wear the effective tilt angle from true horizontal will change. Generally you can expect to replace the rear/steering tires to be replace twice as often. Which means when new rear tires are installed and the front tires are 1/2 worn there is less effective back tilt.
Another factor you need to consider is that the operator must fully back tilt to - in reality lift truck operators don't - they are "too tired" to hold the tilt handle back to reach full back tilt
You did not say what type of loads will be handle - lift tricks hand candle all types including round items like metal pipe or other unstable loads..
There are attachments called load stabilizers or hold downs that might help you - your companies safety person needs to get involved. Contact your local forklift dealer for assistance - you have the drawings, actual loads and other factors you can explain only in words etc of whaat you want to achieve