Since I didn't find this answer anywhere, I took on the task of looking for a calculator and trying to get that measurement, maybe with some luck and everyone's help we can answer it.
I found the measurement of one of them in comment #10
https://www.xs650.com/threads/mikes-xs-rims.61312/post-730695
And the data from comment #11
https://www.xs650.com/threads/front-wheel-not-spinning.61207/post-728318
I used this calculator and got these results.
http://earlymotor.com/leon/misc/html/spoke.htm
Any input is helpful.
I don't have a 21 rim to be able to measure the effective diameter but I guess someone else can.
I found the measurement of one of them in comment #10
https://www.xs650.com/threads/mikes-xs-rims.61312/post-730695
And the data from comment #11
https://www.xs650.com/threads/front-wheel-not-spinning.61207/post-728318
I used this calculator and got these results.
http://earlymotor.com/leon/misc/html/spoke.htm
Any input is helpful.
I don't have a 21 rim to be able to measure the effective diameter but I guess someone else can.