Hi Frisbee,
The yellow box simply cuts into the speedo wire. It counts the number of electrical pulses coming in and puts out some percentage of pulses to correct the speedo to read the correct speed.
It is not really complicated to adjust but it takes some time. The steps are hook up your yellow box. Then go for a ride with a GPS velcro'ed to the take near the speedo. Remember what speed the gps is reading and what the speedo is reading at the same moment. Take those two numbers and calculate the ratio (ie. divide the smaller into the larger). Now refer to the yellow box instructions and set the switches to that ratio. Now go for a ride again and check the GPS against the speedo. They should match. Hint - The higher the speed you take the sample at, the smaller the error will be at high speeds. In other words, a 1 mph error at 20 mph turns into a 10 mph error at 200 mph. But if you have a 1 mph error at 100 mph turns into a 2 mph error at 200 mph. So go as fast as you feel comfortable to get the speeds to calculate the ratio.
Disclaimer - I have not done this yet so my calcs may be a little off.
A I am going to do the same to mine soon so here is the calculations to get close. First correct for the stock speedo error. My stock speedo is about 7 mph faster @ 100 mph than the bike is really going. 100 mph on the speedo is really only 93 mph on the road. That is just about 7% fast. So you would want to adjust the yellow Box output DOWN 7% (Your ratio would be about .93 ... 1 -.93 =.07 or 7%). Your speedo would then read 93 while going 93mph. Cool!
Now to correct the speed for the new 220 mph face plate, we do the exact same thing. Looking at the problem we find that when you were doing 185 mph through the neighborhood before, you are now reading 209 mph (13.07% faster). You are not going any faster and the needle is pointing to the same place but the scale is smaller. So we need to slow the speedo pulses DOWN again. The math is 209/185 = 1.1297, or 12.97%. But remember that is in addition to the 7% you alread have applied to correct the "Suzuki" error. Both are reductions in speedo pulses out of the yellow box so just add them together. 7 + 12.97 = 19.97% Down. The yellow box can correct something like 28% up or down from input pulse frequency so we are within the capability of the yellow box.
This should get you really close to where you need to be as a start. Then you can fine tune it by repeating the process one more time, in case your speedo has a different error from mine. Remember that you have to keep the adjustments so far and just add or substract the new percentage from that. This should get you far closer than stock error without doing the last adjustment. But check it to be sure I didn't screw up my math and subtract when I should of added??
Good luck!