Help needed with understanding accuracy in measuring tools. I want to go straight :-)

[EddyCurrent]

There's nothing wrong with getting more gear but do you need it ? what's wrong with measuring diagonals with a strip of wood having a chisel point at each end to get into the corners ? I'm thinking those squares are quite small so that over a distance of say 1000mm any error would be magnified. There again I've just rechecked the prices and you might as well get them.
There's a 'law' that goes something like this, in your workshop you need 3 things, tools, materials, yourself, but there's always just enough room for 2 of those.