Hello Steve,
Took a look at your spreadsheet. I presume. Like me, I expect you had the method from Ivan Law's book "Gears and Gearcutting". In fact there is an error in Ivan's maths concerning the OD of the small end of the gear. He has :
OD small end = PD + 2 x Addendum x SIN pitch cone angle
This should be :
OD small end = PD + 2 x Addendum x COS pitch cone angle
Likewise the effective number of teeth on the back cone for selecting the No. of form cutter should be N/COS pitch cone angle not N/SIN pitch cone angle
It becomes obvious if you consider a gear of pitch cone angle = zero degrees which is of course a spur gear.
It makes no difference, for mitre gears because SIN 45 = COS 45
Otherwise the spreadsheet is fine.
Regards,