This measurement will depend on the distance the probes are separated, so you will need to be accurate with that distance using your damp towel and the balsa.
What you're after is the resistivity of wood, in Ohms. Here is a handy graph you can use, which covers a range of woods (they all lie between the two curves). So you're getting what... about 1 mega Ohm?
Roughly it looks like you're in a 14%-22% moisture range, based on the graph, which is not for balsa, but for a variety of woods. Triple that resistance would be better. If want to do a really good job, look for about 4MOhm. Resistance of wood also changes based on temperature, but I doubt any cheap off the shelf store probes bother account for this. It also changes if you measure along the grain or across it (as you are doing with endgrain balsa).
Also, I believe the probe separation distance is 1cm for this graph.