This book has the dimensions to cut an odd angle from a long radius 90 degree elbow. From 5 degrees to 70 degrees. From 3/4" to 24" pipe size.
Just layout the elbow in four equal parts (top, bottom and sides) and use the dimensions provided in the book. I use a flexible 3' tape to pull the dimension on the arc of the throat and back outside and sides of the 90 and connect the marks with an old band saw blade or wrap around or shipping banding material.
I've got lots of pipefitting books but I use this one the most.
Pipe Trades Pocket Manual
Thomas W. Frankland
262 pages
$19.95
This site has all the books.
http://pipefitter.com/dolphincart/category.php?cat=Cat0001