This is how I make Pie Cuts for my custom exhaust. No calculations No fancy tricks
Tools required
Something that cuts straight and can make a consistent angle
Bandsaw, chop saw, or reciprocation saw
Marker
Tape Measure
Masking Tape
Welder to glue it all together
Angle iron that is 1/2 of your diameter pipe. (3" you want 11/2 angle iron)