I'm not sure what your post is trying to accomplish xs650skull....
As I mentioned above, with the SR500 wheel and 18/38 gearing the gear ratio is 2.11 (38 ÷ 18 = 2.11)
A stock XS650 gearing of 17/34 gives a gear ratio of 2.00 (34 ÷ 17 = 2.00)
This, as well as every other plausible gear combo, is laid out in the table I linked to above:
http://www.jtsprockets.com/fileadmin/files/jtgearratio.pdf
A stock 16" mag XS650 running a factory spec and very common 130/90-16 Kenda Challenger tire has a diameter of 25.11" and a circumference of 78.88"
A 4.10" x 18" Dunlop K70 tire appropriately sized for the SR500 rear wheel has a diameter 26.73" and a circumference of 83.97"
So despite a gear ratio difference of 5.5% between the 16" XS mag & 17/34(2.00) gearing and the 18" SR Mag & 18/38(2.11) gearing, the overall circumference difference of 6.45% (SR @ 83.97" ÷ XS @ 78.81" = 1.065) between the two, essentially mitigates that difference.
This rear end wont be the ideal fit for every build, but will suite a great many, esp those after a more flat/street tracker look and associated torque oriented performance.
A lot will depend upon the characteristics and tune of your motor, low end vs top end intent, overdrive 5th gear, etc...