Steve Chappel has a great chart in his Timber Framer's Workshop book Trigonometric Ratios pg. 211

under 8/12 pitch roof you will find the numbers .5547 under Sine and .8321 under cosine. You can figure the dimensions of the step lapped rafter seat with these numbers. The given will be the thickness of the rafter tail and the thicker section just before the tail with the adzed reduction. Play with the number. Test them and run them backward and forward to ensure they are right. Of note Sine=rise Cosine=run.

I like how under 12/12 you have the same number .7071, 9/12 you have .6 and .8 respectively. and 8/12 cosine is .8321 fun numbers and easy to remember. Just divide or multiple but your givens, rise or run.