The most useful tattoo?

After last weeks harrowing lobster debacle, (yes there was tears, and it was delicious, I will post pictures next week) I've harnessed my grief, and attempted to move on by focusing my attention elsewhere. This week I've been searching for the most useful tattoo.  There seems to be alot of ruler tattoos out there, and a lot of science tattoos, but there's not alot of tattoos that do anything.   I put plenty of thought into it, and here's what I've got so far.  

There's a ruler in inches and cm, and sort of free form protractor, (which can also be used to measure pipe diameters, although that's a bit redundant, because it's right next to a ruler) there's also lines for writing and prioritizing n

otes, and a rudimentary RGB/CMYK color wheel. 

 An earlier version had a compass around the dish of the elbow, so you could put water in it, and add a magnetized needle to make an elbow compass, but that was removed on the grounds it wasn't useful enough.  I'm looking to revise and expand, and I've developed some rigid stipulations:

• It can't be any theorems, or reference materials, say like Fahrenheit to Celsius conversions.  With reference materials, it's only handy for a month or two, then you've memorized it, and it's taking up valuable meat space.   
  
• Measurement devices are useful, but the detail is restricted by width of the tattoo needle, and the tendency of the tattoo to distort and diffuse over time.  

Some ideas I don't know enough about to incorporate

• circle of fifths
• star charts
• vital identification information, i.e. blood type, allergies
• camera calibration information
• semi-functional slide rule on there for taking logarithms (Thanks Anon!)
  

Please help!  I want to hear from builders, tattoo artist, graphic designers, fry cooks, anyone that has anything to say, I want to hear it!

EDIT 4/2/09

Here's an Ethernet cable wiring diagram, as requested.  I put them in octagons so they can extend off of the one inch marks, and won't look out of place if more are added later.   The yellow contrasts poorly with the white background, but I expect it would be highly visible on skin.  I will add a diagram incorporating modular elements after I get a few more suggestions.  

I've been emailing Mikey Sklar, who's had a ruler tattoo on his arm for two years

 now.  He is my unwitting de-facto beta tester, and he was goodly enough to give me a few suggestions:

Things I would have done different:

- tattoo goes on non dominant forearm. I am right handed and should have had the tattoo put on my left forearm. That way I can hold tools in my dominant hand and measure with the left.

- consider a additional index finger tattoo in metric and English units. These are good for small measurements in places you cannot get your forearm into.

- two circles for tea spoon / table spoon measurements on non dominant palm.

- during tattooing do not flex your wrist. My ruler goes too close to my wrist and I fidgeted around from the pain. This makes my first inch off unless I recreate the extreme flex my wrist had during the tattoo process. 

I like your pipe sizing idea. Looks useful.

My ink and numbers look good still. I use sun screen on it. I will mail you a photo tomorrow.

As promised, he mailed over a current pic for me.  On the bottom you can see the freshly done tattoo, and on the top, two years later.  It looks to me as if the ink spread enough to make the 1/16th marks nearly indecipherable.  I expect making the different increments with different color inks would keep it more legible after the ink begins to spread, but I'm gonna run that by some body mod forums, and see what they have to say.