i wouldnt try to over-think this too much
safety is your main concern (even over money)
if you have sitdown counterbalance AND standup reach trucks negotiating this ramp the obvious calculation would have to favor the standup reach truck and building the ramp so that these trucks can negotiate this ramp in the safest manner. Reach trucks being the most likely unstable on a ramp situation.
Even basing the ramp angle on what the spec's say the reach trucks can do is tricky because you also need to factor in human interaction (the operator's judgement and ability to operate the lift even if trained) IOW... no 2 drivers are the same so the safer you make the ramp the better off you are.
The lesser the angle the easier it would be to negotiate the ramp safely.
I've had some experience with ramps on reach trucks and it is a very tricky situation when carrying loads and even more so driving empty. Trying to negotiate a ramp on the down angle on a reach truck is tricky especially if the truck has electric brakes. Sure you can use plugging to help slow the truck and control it but the steeper the ramp the harder it is to achieve this in a safe manner.
Not to mention the effect it will have on the battery life of the truck, steeper ramps take more power to pull the truck up the ramp.

Just some of my thoughts on this
Just remember safety for your operators is your main concern period.

good luck