General Algorithm for Primitive Calculation?

[Philippe Hurbain]

(You don't want to round anything except the final points written the file.)

That's not completely true. As Nils guessed, sin/cos of ring and cones primitives must be rounded before scaling them otherwise they wouldn't match other round primitives.

and round the result again to four dp (how should I round?)...

AFAIK it's simple rounding to nearest value. I beta tested Primgen2 and we struggled quite a bit to make it compatible with the original Primgen (for the sake of completeness Paul Easter Primgen is here http://www.geocities.ws/pneaster/prim-gen.html) while extending it and correcting its issues (mainly make the edge condlines tangent to plane/cylinder for cones and tori). I hope Mike will chime in but then... it's holidays season