Hi:
Without adding to your anxiety, this is my 2 cents worth,
gathered from months of asking around and studying tapes and a review course:
Yes, every course gives different measurements and this could be quite confusing. But the fact is, examiners rely on visual assesment of the preps, and they won't spend more than 20 seconds to look at each of them. Rarely will they measure the amount of reduction, because they have looked at hundreds of preps in their carreers as examiners, and they already know what to look for. They would cite your work for over-reduction, not too much for under-reduction--that, according to what friends tell me...At least one reviewer told me to be conservative and NOT to cut to the required reductions. Leave a lot of room for error and adjust later as necessary. That means, under-preparing a litttttllllle bit then approach the desired dimensions of the preps during finishing.
There is also a difference between *reduction* and *clearance*. Even well prepared preps on a well equilibrated typodont would look over-reduced even if you cut say, 1.5 mm on the occlusal (which is textbook figure). How? Look at the disto-buccal and distal cusps of 19 and 30. Even on a well equilibrated typodont, there already exists a clearance here against their respective opposing teeth.
Cut the required 1.5 mm occlusal reduction here and you fail for over reduction. You say "hey I cut only 1.5.." but the clearance would be around 2.0mm--a clear failure.
Also consider 6 and 11...on many typodonts 6 looks longer than 11, so for a PFM, you might want to adjust the cervical margin coronally for 6 so the prep won't look too long.
I'm sorry if this posting caused you more confusion. I am as confused as you are...
😀
Ivorinedust
