Fidelity Research FR-7 Repair

"The protractor'', ''an protractor'''or ''whatever protractor''?
The tonearms lengths are 9'', 10'' and 12''. They ''assume''
different effective length. Aka the stylus distance to the pivot.
Letch owns 10'' tonearm (FR-64) while eff. length is 245 mm
for Stevenson and 246 mm for Bearwald. If his FR-7 got the
cantilever with the same length as the original this length can
be used for whatever other carts and headshells. Assuming the
headshell has slots allowing movement of stylus in both
directions. My guess for the cost of an plastic caliper is $10.

@nandric technics overhang gauge cost $7 , look here
Alternative mark for the stylus position (overhang) on the side and you’re done with that. At lest this gauge hold the headshell like a tonearm, look here. I tried it with y FR-7fz and it's fine, the stylus is a few mm behind the mark on the gauge. 

Sorry chak, I see an rubber ring behind the headshell connector
while the higth of this ,uh, cheap instrument, does not allow
lowering of the stylus to the marked position. One need to guess
where the stylus point should be. Together they make at least
2 mm error. So there is then no sense to speak about different
geometries.  But if one use (plastic) caliper with one ''leg'' after
the connector (without rubber ring) and the other on the stylus
point one will get exact effective lenght  and use the obtained
lenght for all other  cart/headshell combos. I adjusted this way
20 of my MC carts in their (separate) headshells . Checking with
my Mint tractor made for my SP10 /FR-64 S (Bearwald) confirmed
the results. Changing carts can then be done in 5 minutes time.

I don't use any rubber rings and the pictures provided above are from the internet to display a $7 the plastic overhang gauge to the OP.  

So dear chakster, You can sell your Feickert  and buy one plastic
caliper which you can use for all your carts/headshell combinaions.
The only thing you need to separte are your tonearms and headshlls
which you use with them.