Here's what I think should be done in theory:

1. Up to 65535 RWIN is a multiple of MSS
2. According to RFC1323 large windows are scaled by multiplying the original RWIN by some power of 2

So, then, assuming MSS of 1460, 44 x MSS = 64240 (highest multiple lower than 2^16) It might be a good idea (although not set in stone) to make it an even mulltiple, so slow start and congestion control algorithms can benefit from it (not sure about that but it wouldn't hurt anyway)
When scaling is turned on, we could use for example 64240 x 2^2 = 256960

This number would be the Max RWIN, while the current open RWIN sniffers report for every packet is going to vary. Any thoughts on my math ?

rmucker, here's a sniffer program you could use: