The front of the cowl isn't an exact circle, so I found it easier to stuff a rag between the upper cowl and the engine as a rest and then to align everything manually. I measured from the engine centre tube to the sides and top to bottom until I was sure the cowl was in proper alignment (checking the twist as well), then marked the aft edge for trimming. We did this a few times until the positioning was perfect and trim line very close. When that was done, we marked out the spacing for the camlocs in preparation for drilling. The plans call for 4" spacing, but that corresponded to too many things below the firewall flange, so I closed the spacing to 3" and it worked out well on both sides.