I talked to a guy that didn't want to drill the rear of his C4 case for the additional oiling (#9 oiling mod) because finding the right spot was intimidating. I understand that and measured about 53 times before I drilled my first as well. Anyway, I have a simple method to locate the drilling spot that is accurate and fast even for those comfortable with the process, so I thought I'd share it here.
You use a #9 thrust washer for a C5 as a guide. They are available from most suppliers and may be the only type you can get anyway as the original C4 part is obsolete. For example, Bulkpart.com has them for about $2. You would want the C5 part in any case, as it has twice the lube channels that are also twice as wide. Much better. See the pic below which has a C5 thrust washer on the left and C4 type (previously modded for oiling) on the right. Sorry about the picture quality - my little camera lost it's macro focus capability so they're a little fuzzy. I'll try to get clearer ones to post.
Now, here's how it works. The #9 has a bit of side-to-side and rotational play and we want the new oiling hole exposed no matter what the washer's position. Since it's hard to get clear photos of the rear of the case - here's a drawing of the rear of the case with the tab cutouts top and bottom that I'll use to show the process:
So we take the C5 washer, place it in the case and rotate it to the right, while pushing it away from the cooler line fitting (away from the little hole already in the case) like this:
Use a marker or a scribe to outline the U-shaped cutout while holding the position. I used a pen here:
Then, while still holding it away from the cooler fitting, rotate it to the left and scribe again:
You should now have two overlapping U-shaped scribes like this:
The spot to drill is at the center of the double-U just inside the lines like shown here:
Now you should be able to verify your placement by sliding a drill bit or rod into the lube passage and eyeball the alignment of the bit, the little hole already there, and your marked spot. If everything looks good, I center-punch the spot, and use a Dremel on low speed to drill the hole. The drill should penetrate and drop into the lube passage pretty quickly - about 1/8" or so. Put a wrap of tape or a bit-stop on your drill bit at 1/4" (almost all the way through the lube passage) to be sure you don't drill deeper than the far side of the passage. Use a bigger bit to deburr the hole and be sure to get all the little shavings out. You're done!