After first determining that convergence is roughly correct, checking purity is really the biggie. You don't want to do a
full convergence procedure without first having good purity, because purity adjustments interact a bit with convergence. On Maggie roundies, purity is done the same as the RCA procedure. And it's done with the red screen.
With the set out from the wall but in the same compass position, turn the blue and green screens all the way down and the red screen all the way up.
Give the set a thorough degaussing.
The red screen should show perfect purity. If it doesn't, loosen the yoke clamp and slide the yoke as far back as it will go.
Use the purity rings to center the red blob as near center as possible.
Slide the yoke forward slowly, watching for the 'sweet spot' where best purity occurs. It will be with the yoke roughly 1/4" to 1/2" shy of fully seating against the CRT bell. Clamp the yoke at that spot (while being sure to level it also).
This point may or may not be covered in Sams, but it is the RCA factory procedure. (It applies to 25" rectangulars also, but with the yoke's 'sweet spot' 1/8" to 1/4" short of fully seated.)
As the previous poster mentioned, feel free to repeat as practice makes perfect.
Bill(oc)