How do you figure ridge height

Depends on the pitch of the roof. But, any height you want. Typical house is 4/12 to 7/12 or steeper. Determine the width, divide by 2 and then determine what pitch looks good. For example, if 1/2 width is 12', a 4/12 ridge would be 48" high, 5/12 = 60". Too steep and you can't walk on it. Too low and you can get water under the shingles. Older ranch houses we're typically 4/12. Newer Craftsman style are 7/12.

Also. for measuring cuts, there's plenty of 'roof pitch calculator' sites online for inputting any variable you want to get the other calculations. You can even choose the kind of overhang.