I used 500K, because it was more easily available when I ordered the parts, but I would use 1M if available. Trim pots were adjusted to 50% before installation.Was it 1M trimpot or less?
The board is laid out so that turning the screw in one direction on both sides has the same effect on the offset voltage. In other words, the wiper-common on the positive rail is opposite the wiper-common on the negative rail.
If the intent is to just set the offset once and leave it, Shaan's method is best (and costs less). I was not that confident, and wanted to see how the offset changes with temperature, and after a few hours of operation (answer is, very little).