Keeping the bubble between the lines means you can be out of level about 3mm over 1 meter. This is the absolute minimum as far as acceptable quality control goes. To be "half a bubble out" is about 30mm over 1 meter. Oddly enough my 100 year old American antique mostly agrees. Maybe there is such a thing as bubble standardization. (And yes I collect measuring devices. I know, its weird, please don't tell my girlfriend.)