Tyson, I have collimated about three of these scopes. The first thing to know is that the secondary is minimally sized for keeping the CO small. What results from this is that there is vignetting at field periphery. This will show as out of round fresnel patterns toward field edge. With my MN-66's this started almost the moment the star was off center. So bear this in mind when collimating.
Secondly, the M-N is different from a plain Newtonian in that, in the Newt, the aperture stop is at the primary mirror while in the M-N, it is at the corrector. Consequently, focuser tilt is not tolerated as it's not possible to tilt the corrector. The primary-secondary-focuser optical axis has to coincide with the corrector axis as well.
I would start collimation with first making the secondary as parallel to its holder (screwed into the corrector) as possible. Then adjust the focuser up/down (the better M-N's have this adjustment) till the secondary is centered (but may present as oval). Next rotate the secondary by first loosening its retaining ring. There is a tiny set-screw in the retaining ring which has to be loosened first. Once you have a circular looking secondary in the focuser, you can proceed with the usual Newt collimation steps, iterating if necessary. My primaries had all a plug in the center so I could never use auto-collimation, but using a cheshire, I could successfully collimate them to perfection.
Since these are fairly short focal length scopes, using an artificial star, with extension tubes for the eyepiece (I used barlows with optics screwed off), will be a good idea to check the final primary collimation.
Finally remember that I-M offsets the secondary a bit so it may not look centered after final collimation.
EDIT: fixed typos
I just checked this out with the astrosystem's combination tool. Everything looks really good. I think this may be vignetting, that is only noticeable at higher powers?
I didnt see any light fall off with a Pentax 40 XL EP, or 27 pan. Just the 10 Pentax and above, right after the star gets off center.