As a quick check if you're in the field and unsure of how big an error rounding the degrees will give, I recommand doing a projection witht the two nearest whole numbers. So, in your example, a projection with 58 degrees and a projection with 59 degrees. If the resulting waypoints are close enough together, it gives you a line to search in between. If they are far apart, you'll know it's worth doing the projection on a computer with geocalc or a similar tool...