my lighthouse plan calls for a 68 degree cut on the roof how do i do a 68 degree cot on 3/8 ply wood clayt
Question not real clear. I'm guessing it is a round light house. Is that around the circumference so it would be an 8 sided piece or the roof pitch? If being the circumference it would really be the opposite of 68 which is 22. That comes from a starting point of 90º - 68º = 22º. If 8 sided the cuts would be 22.5º. Added all together it would equal 360º (full circle).
If for the roof pitch (taper)I would make a simple tapering jig to use with the table saw. It would still probably be a compound cut so you would need to tilt the blade to the degrees needed to make a circle when glued together. The math to make a complete circle is 360º ÷ (number of sides) gives joint degrees. Divide that by 2 gives cut degrees for each side of joint.
Example for 8 sides: 360º ÷ 8(sides) = 45º ÷ 2 = 22.5º
Your cuts will be 22.5º on each end of your pieces. Put together you will have a complete circle. The length of each piece would depend on the total circumference that you need.