Just as in the A-I process, once you learn the basics, it all comes down to the numbers. Gentle, consistent technique is KEY. The more you get into and see the large differences -- even at the same gestation time frame from cow to cow -- the better you will get. It all comes down to one thing -- from the very simple things to the more complex -- it is ALWAYS the cows that teach -- NOT the books. The books are for refference -- of course every one needs refference -- but the cows will ultimately be the teacher.
Start simply by guessing -- then using an accurate A-I record to follow you up on your guess. At calving time, use the actuall calving date to back date what you called her at the preg check. When the light bulb comes on, it usually stays on!
Best of luck -- life is good! (Albeit damn expensive this year)
Terry