If using a wire or three wires to measure the pitch diameter, be aware that any error in the actual diameter is multiplied be three in the pitch diameter calculation. For accurate measurements, it is important that the wire be round to within a tenth or two of a thousandth. To that end, baling wire would probably be a poor choice, at least without pre-qualification.
Secondly, the wire diameter should be between 60% and 90% of the thread pitch. If it's too small the wire won't stand above the crest of the thread and if it's too large, the wire won't be making contact within the flanks of the thread.