BioNMR - Decoupling, what modultaion frequency to use ?

Hi all,

Perhaps a stupid question, but so far no one answered this one to me.

I want to find out what decoupler modulation frequency (dmf in Varian syntax) is needed to properly decouple observed couplings. Can anyone tell me ? I am looking for a practical answer.

I found a paper on decoupling, i.e. Shaka, A.J., Keeler, J. (1987): Progr. NMR Spectroscopy, 19, 47-129. Although this paper is formally correct, it is also very hard to read.

So let's make a practical example:
On an 800MHz machine, the carrier is on water (4.5 ppm), and there are imino protons resonating at 12-14 ppm. (JNH = ~40 Hz)
I want to observe nicely decoupled signals for these imino protons.
What decoupler modulation frequency is required to properly decouple these protons ?

P.S. I realize (of course) that the pulse-power has to be adjusted as we decouple.
(If requires, assume a hard 90-degrees pulse takes 8 us at 60dB)

Regards,
Ramon