There is a site that describes how to determine spoke tension by plucking, follow a link from Sheldon's wheel building site.
Different lengths have different tones. These would be for a full number of spokes.
For a lesser spoked wheel, tension has to be more, a simple first stab at it is to take the ratio of 36 spokes and your wheel's number, and multiply the tension. Taking the small wheel diameter into account, multiply the tension again by the ratio of the smaller and 700c wheel. So a 24-spoked 20" wheel's tension is equivalent to a 700c 36-spoked wheel's. It's all about the distance between spokes at the rim.
Thanks for the tips. I do have a Park Tension Meter and wheel building stand, so can be a bit scientific about it.
From what I've read, the recommended tension is determined by the rim manufacturer and can vary quite a bit within the same wheel size. As I don't have a known properly built wheel to get measurements from, I need to find the specs from the manufacturer.
I've emailed Alexrims, so hopefully will get an answer soon. Had been hoping in the meantime that someone here might have the answer so I can get the wheel fixed for a ride today. The sun is shining and da boids is choiping. As luck would have it, a local shop had a spoke machine so was able to come up with an emergency replacement.