Many thanks for starting this discussion Ed!
Hopefully there will be many contributions; reducing the harsh lessons taught by Experience
Here is one on one squareness:
Compared to the blade, the stock of a 600 mm (24″) combination square is fairly short. When checking for end to edge squareness of 450 (18″) wide pieces, I didn’t notice that on out of square piece even very slight pressure on the blade would have the stock angulate, allowing the blade to come flush with end of the piece, and the false impression of squareness. It was only very slight, but as these errors grow over distance, my concealed mitred dovetails were short of completely concealed.
On the very same chest of drawers (bottom piece of a “hutch”) I used the combination square to outline sliding dovetails and housings /dadoes for the rails and runners, respectively. This time I wasn’t observant on that the lateral pressure of the knife on the blade was enough to actually move the blade, despite the counter pressure I applied to the stock – or thought I did. Levelling the rails and runners gave me a new understanding of the concept: necessary tools. I also had interesting times fitting the drawers.
When checking for squareness on subsequent projects, I have the stock of the square secure against the edge, and with the blade resting on a strip of wood I move it towards the end of the piece. It works very well, and there are no problems with squaring most pieces, except those more than 400 mm wide. Thankfully, the timber yard has a very accurate saw and equally helpful staff.
For laying out I’ve turned to the time honoured method of a setting out/layout rod; using a mechanical shop rule to set out the distances. Then, using a combination of a clamp, a square base (T-base) and a straight edge I make the lines. It worked very well for a metre tall cabinet, which came pleasantly different from the one it rests on.
London, UK; Boston, MA