Handicap ramps have to meet certain requirements by the US Americans with Disabilities Act. Having an incorrect angle will not only violate the federal requirements, it will also make it very difficult for someone who needs the ramp to actually use it. Finding the correct angle isn't difficult. Here's what you can do.
The ADA set a maximum slope of 1:12 for a ramp that will be used by occupied chairs. This means every inch of rise will require one foot of ramp length. The number in inches is what you will need in feet of length, so a 36 inch rise will require a 36 foot long ramp.
The only exceptions to this rule are that ramps with that only go up a few inches can be slightly steeper, with the legal maximum of 1:8 for ramps that only go up three inches, or 1:10 for ramps that only go up six inches. Ramps of any height steeper than 1:8 are not allowed under the law.