I have a second arm on my Garrard 301. The second arm, a Thorens TP13A, mounts at 215.6 mm and the primary arm, a Grace G707, mounts at 222 mm. 222 is 8.74 inches and 215.6 mm is 8.48 inches. Remember, there is about 15 mm of overhang on Grace, and about 18 mm of overhang on Thorens, so effective length of Grace arm is 237 mm or 9.33 inches and effective length of Thorens arm is about the same, 233.6 mm or 9.19 inches. "Effective length" includes the overhang. 202 mm mounting distance seems short, unless arm is really short. That's a little less than 8 inches mounting distance.
I agree with rorenoren. Measure the mounting distance, spindle to center of mounting hole for your added arm in its current location and use that as your distance. Using a figure cobbled in from formula may not give correct distance. You may be able to cheat a little by using the "effective length" from arm pivot to stylus and subtract the overhang.
You need to consider some things when mounting a second arm. First, second arm should be accessible. It's not easy to figure this out, as you can see from my 2 arm rig. Also, LPfan is right. You may have to take springs out from under turntable, and tie turntable right to new plinth, since the second arm must not be allowed to change its distance from spindle as turntable goes up and down on springs. This will induce distortion, and maybe speed irregularities. Didn't have to worry about this on Garrard because it bolts directly to plinth.
You will also need to consider grounding on second arm, My experience was that I had huge hum until I made sure that the right and left channel grounds were isolated from each other and also isolated from turntable chassis ground, which came in on a separate wire. I also had to be sure that all chassis ground wires and cartridge grounds were isolated from each other till they got to preamp control unit chassis and connected up there.
I designed an isolated ground switch that I used to stack both arms on one input, (2nd input on my Hafler preamp takes a Thorens TT with a Linn arm) and it works well. It has become a somewhat popular item on VE. I will send pix and a circuit diagram for the switch if you want it, but you will have to PM me with your Email address because VE will not accept PDFs or pictures from my computer. Also if you want pix of 2-arm Garrrard, say so and I will send them too.
And good luck from the old guy with too many arms,