you can make them what ever angle you want....just make sure that you'll have enough room between the notch and the axle to lay frame with your bags n stuff....and remember your not baggin it on 30's so the notch doesnt have to come up past your bedrail. okay maybe that was a bit of an exaggeration but you know what im saying..
you can find out exactly what angles to cut and the different lengths as long as you have two values. with at least two values you can find the angles of each cut and the legth of the angle peice using the trig functions such as SIN or COS depending which way you do it. you can break the angles into triangles and solve them separatley then do the math for the straight piece. you can also use the pathagorean theorem. (A^2 + B^2=C^2)
your gonna have to do the math with the size wheel your running. take the overall diameter and divide it by 2. that is the center of your axle. then get a tape measure and measure from the bottom of your frame to the ground. use that info to get the height of the notch. (one side of the triangle. A) take into consideration your bag height deflated plus bracketry if you plan on putting your bags over the axle. approximate the overall length of your notch. lets just say 2" in length. which would give you your 2nd length value (A^2 and B^2) in a 90degree triangle. plug the two values into the pathagoreon theorem to find the thrid value. C^2. now with those three values you'll be able to use the trig functions SIN and COS in some instances dependiing on how you use them to find the angles. SIN is opp. / hyp. COS is adj. / hyp.
this is the best i could do to explain it...so hope that helps you out. i built a trebueche (sp) a while back and had to use this to get the values for it for my design. i can post up the drawings i did with the work on it as to how i found the lengths and angles n stuff if you want it.
6" notch is sufficient to clear 20s right or is an 8" notch, im just checking, i dnt know if im going 20s again or not, i just want to know before i notch