Get a 6" deflecting bar tension wrench
I still have the one I bought back in 1968 and it is still accurate
Spring loaded ones must be backs off to 0 every time they are finished with because the spring will shorten if left compressed , just the same as a valve spring will develope a set even in an engine that has not been run for 20 years .
I think just about every BSA club member has borrowed it at one point in time
You should realistically avoid using the top & bottom 10% pf any scale and a 6" or even 8" will have you reading the 20 to 30 ft lbs fairly well mid scale
Also the shorter leverage gives you better feel for the all important method of tensioning a BSA
Tighten till the thread goes loose then back off 1/2 a turn
Warre & brown are still marketing their signalling deflecting bar tension wrenches which is what I have.
One under their brand & the other branded Repco .