This gleaned from the internet may help:
“Maximum slope for hand-propelled wheelchair ramps should be 1″ of rise to every 12″ of length (4.8 degree angle; 8.3% grade). Maximum slope for power chairs should be 1.5″ rise to 12″ length (7.1 degree angle; 12.5% grade). Minimum width should be 36″ (inside rails) – (48″ is ideal)”
So 1 in 12 for hand propelled or 1 in 8 for powered.