Thanks again, everybody. Hope the techniques I'm using will come in handy for someone.
Chris,
Regarding the overall dimensions and proportions: I haven't found anywhere that's off by more than a couple of millimeters here and there. You're right that it's hard in some areas to translate 2D pics of the real deal to a 3D sculpt, but everything's been lining up extremely well.
When I do my "transfers" I take a known measurement of the helmet, say the width of the keyslot area, and scale the pics on my monitor so they're the same size. Then I trace the damage, and transfer it to the helmet. Doing it this way, the only areas so far I think that have been proportionately a little off are the corners of the T-visor. Like I said, it's only a couple of millimeters, so it's hardly even worth mentioning. Everything else has lined up just about perfectly.
UPDATE: November 23rd
Two steps forward, and one step back.
Not sure what happened exactly, but somehow I managed to make a pretty big error in transferring the damage area around the upper left corner of the T-visor. Everything in that section came out compressed somehow. :confused I noticed it when I was painting the damage in and around the killstripes.
Here's the area I'm talking about. It's the mostly silver area just below the most broken killstripe.
It took me a considerable amount of time to figure out how to mask things so I could respray my mandible color without losing all the work I've done so far. I finally managed to get everything masked off, and used some fine sandpaper to smooth out the silver/gray/maroon areas I had painted. Once it was smooth, I crossed my fingers and sprayed the area again with Boxcar Red enamel.
After removing the masking, I was left with a nice undamaged surface to repaint.
After allowing time for it to dry, I remeasured the damage and redrew my transfer, double checking as I went to make sure it would look correct this time.
Now that it's repainted, it looks MUCH better. Here's a side-by-side of the before and after.
Prior to working on the repainted area, I had finished detailing around the dent and the damage all around the killstripes.
I still have to go back and do the very fine scratches in these areas and then the big streak coming back from the dent. After that, I'll paint the big weird scrape on the top of the helmet, and then all the main painting will be pretty much done!
I'll still have the ears to do, so that'll keep me busy for a few more days.