I just picked up a set of 993 16" wheels (7", 9") with an offset of 55mm.
http://www.wheelenhancement.com/99316.htm
So should I get 32mm spacers front and back (55mm-23mm=32mm)
....or should I put 25mm up front with 32mm in the back? (Using the guidelines on the racer_x site)

It seems that some people have problems with the center cap fitment. So, should I add a couple mm to compensate for this?

I've been reading for hours...and I am now throughly confused. Thanks for any help!!

 

55mm isn't far off the 52mm of the "normal" conversion - so I wouldn't worry about the extra 3mm. Do an inch conversion and you'll see - it really isn't anything at all. As such, I'd just use the standard spacers in the conversion charts. For my 86 951, I used 21mm spacers at the front and 28mm spacers at the rear. Both spacer sizes are standard Porsche parts and readily available. The 21mm spacer is from the rear of an early 944, and the 28mm spacer was used on 911 (but I don't recall when). Adding extra width on the spacers will not cause the center cap fitment issue to go away. You, or the shop that installs, will need to do a small amount of grinding on the front spacers to get them to fit.
I normally keep a chart handy to post with all of the part numbers, but everything I own is currently in storage and it's not on my work computer. Do a search on the topic and you'll find quite a few posts where the part numbers are given.