Here's the spreadsheet graver angle calculator for those interested. Just type in the the kind of graver you want (90°, 105°, 120° etc), the heel angle you want (15°, 20°, etc), and it will calculate the sharpening fixture rotation angle to produce a parallel heel graver.
Graver Angles by Steve Ritter & John Henley
Graver Angles by Steve Ritter & John Henley