i am assuming you are talking about building up an engine with new parts here, if it's something you just stripped then the simple answer is that the rings are worn.

since you mention haynes manual, i presume you are aligning the rings by pushing them in with a piston as they usually describe? if they aren't in straight you'll get a big reading.

1mm is too big and will give excessive blow by.

did the rings and pistons come from the same source? sometimes the tolerances on aftermarket pistons are a bit ropey, and the recon guys tend to bore to the piston size as it's easier for them to measure clearances than accurately measure the bore size. gives a good fit between piston and bore but if the ring comes from someone else......

having said all that, controlling the ring gap tolerance is not normally too difficult, hence the first question about the measurement.