I have a MTU of 1478. RWIN is supposed to ideally be a multiple of MSS (which is MTU-40). To get my RWIN I used MSS 1438 and multiplied this by 12 to get an RWIN figure of 17256. This was for half meg.
Whe I upgraded to 1 MEG I doubled the RWIN figure (so is now MSS x 24) to get 34512. I now get exactly twice the speeds I was getting before.
Try running a tweak test from click here and then have a look at click here
If your MTU is 1458 then MSS is 1418 so experiment using multiples of that.