Ride height alone is not the way to determine this. As an extreme example of how not to do it, assume you want the trailer sitting as low as possible. You select a zero degree axle and you now have the frame sitting nearly on the ground. The swing axle needs to pivot through it's normal range of travel so in this example, the frame literally would hit the ground on each bump. The major factor in this selection is whether or not the trailer frame is parallel with the road (not al of them are). If so, draw a horizontal line on a piece of paper. Using a protractor, draw angles off the frame at 15, 22 and 45 degrees. One of those three (probably 22 degree) should work for you. If the trailer frame is NOT parallel with the roadway, then start with a horizontal line representing the roadway. Then another line representing the angle of the trailer frame in relation to the roadway. Now draw the various swing arm angles off the trailer frame line. Remember, it is the swing arm angle in relation to the axle mounting pad and the pads relation to the trailer frame that determines what angle you need. You might stop by a marina or dealer and look at a few trailers to see what the angle is on those.