all valve jobs should have three angles. The norm is 15 degrees from the seat angle. So a 45 degree seat gets a 30 top angle and a 60 degree bottom cut. The main reason for the narrowing angles is to control the width of the seat, and control where the seat contacts the valve face. The valve can only really dissipate heat when it contacts the seat, wider the seat the cooler the valve runs, narrower the seat the hotter. Generally as a rule the narrower you can make the seat and it still live the better the flow. Depending on how deep the valve seat is in the chamber and how the chamber is made, another top cut is done usually around 15 degrees. I even use a 70 degree some times as another bottom cut. And just because it improves flow on the bench does not mean it will make more power on the track. I seldom back cut a performance style replacement valve. On stock type valves the back cut seems to help on the intakes the most. It is just one of those things somebody has to spend the time and money to actually see what works the best. And as a rule the more you grind or cut the seats the less HP they will make, because as you cut the seats they sink into the heads, because you are taking material off. And the more they sink they loose flow, at least that is my experience. The main thing is the valves have to hold compression, if they leak, no matter what the seat flows, it will just not run right. I use a vacuum tester and a leak down tester as I am assembling to make sure they hold air. On a ported aluminum head if you over torque them for example, I have seen it warp the seats and they will leak compression. And I do not know how many times I take a new head ready to go out of the box and the valves leak air.