[ccp4bb] equivalent osmotic pressures for xtal transfer

[Frank von Delft]

Hi all - The Google fails me, so I'll the CCP4BBoogle:
Can anybody conjure up some of the references that describe the details 
of how to get a (say) PEG solution to have the same osmotic pressure as 
a (say) salt solution - in particular, this is for transferring a 
crystal from it's original salt condition into a more ligand-friendly 
PEG condition.

I know there was something from the late nineties, where specific lookup 
tables were either shown or mentioned;  but I've not been able to track 
it down.

Thanks!
phx