[Neuroscience] Series resistance

[Hyunchul Lee]

So, here's a question-

How does one calculate series resistance in whole-cell configuration?
In a recent paper by Lee et al. (2006), I noticed that though they used 
patch pipettes of 4-6 MOhms resistance,
their series resistance ranged from 15 MOhms to 80 MOhms.  Is it a 
simple matter to do that in voltage clamp?

Thanks,

Hyunchul