That was always my big problem with it.... No one has ever given me a good explanation for why you have to do it.

I cheat and always have a little chart taped to my kneeboard that has the GS and the pivotal altitude for that GS. I might have taken it from the Airplane Flying Handbook, but I can't really remember. Anyway, it definitely has helped me because when I enter the manuever I just look at my GS on the 430 and then look at the chart to determine my pivotal altitude to enter the manuever. Then I might glance at it during the manuever as my GS changes to see if my altitude is what it should be according to the chart. It has helped a lot.