To calculate the ridge length for a hip roof is pretty much a straighforward affair. In the example roof, the building length is 75' and the width or span is 32'. This would give us a run of 16' or192". Subtracting 192 twice from 900 (75') would give us a ridge length of 516". This works well if your roof is a simple rectangle, but a better method might be to find the location of the ridge from a hipped end. Measuring in 192" form the hipped ends would give the location of the two end points thus measuring between the points would give the ridge length.

The last thing is to adjust the ridge length. The ridge will "grow" by 1/2 the thickness of a common rafter. In the example the commons are 1-1/2" so the ridge would grow by 3/4" on each end. Or, the location of the ridge end point would be 3/4" less then the run.