I recall once reading that the maximum slope you'd want for a motorcycle ramp would 1 in 8; that is, for every 10cm up, you want to go 80cm forward. That seems quite flat-ish to me, but if you don't want to be struggling with transitions and so on, it might be a starting point.
How about one of those folding purpose-built motorcycle ramps? For about a hundred bucks, you might get everything you want without much effort.