I agree, that for adjusting the focal length, that I would start out at least with
C-band. With C-band, the center lobe is more gradual, but there always *IS* a center . With Ku, if your focal length is off a bit (and it doesn't take much), instead of getting a center peak with some side lobes, the center lobe can dissappear, and all you are left with is the side-lobes. Ie a donut shaped pattern. Because of this, you can often be fooled, and focus on the side lobe rather than the center lobe. When you do this, you can get big signals on Ku, and yet be significantly off in the aim on C-band. I have done this several times, and it gets confusing. Since C-band generally gives you only the center lobe, I'd peak on C-band first, then see if you can touch up the adjustment with Ku if necessary. C-band and Ku *SHOULD* focus at the same place. If they don't, I start to suspect focusing on Ku sidelobes.
Adjusting the focal length is the hardest part of aligning a dish, as far as I'm concerned, particularly with dishes with quad arm feed supports. I've tried many times to peak the FL, but usually end up just using an tape measure and using theoretical measurements.