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.