I have found that most slopers are using 105 degrees to create the V. I just finished a fuse and V tail for a surplus wing. I didn't research the formulas. I just measured the span of the horizontal stab and added 1/4 of that figure to each side of the vee. I kept the cord length the same. I swept the tail back 75 degrees. At the control surfaces, I added 1/4 as well. The plane flys very well and is very responsive.