If you're worried about preps, which it sounds as though you are--then STOP. Cutting preps on ivorine teeth is nothing, in my opinion, like cutting them in the mouth.
The beast known as the 'ideal prep' really doesn't exist much in actual dentistry. More often than not you'll be chasing decay. Sure, you have to know what ideal is, and know how to drop a box, and know about retention, and know about axial walls, etc., but you'll never combine all of those points into one single prep--or at least not unless you're prepping a virgin tooth.
Once again, don't stress it. That's my two cents.