Take your average latency, which is ping + 50% (in the event of slower lines), multiply that number by your proposed (advertised) speed and divide that value by 8. You have your raw RWIN.
Example: 50ms average latency x 1.5, with 1500Kbps available (purchased) downstream bandwidth:
75 x 1500kbps = 112500 divided by 8 = 14063 RWIN.
However, when dropping RWIN below 8192 (which is Windows default for Windows 95/98/NT, while Windows Me and 2000 use default of 16384), try using even multiples of MSS. Your MSS is listed on the Tweaks test.
For more information please refer to this page.