RE: Inches to milimeters
I don't think that 0.1mm is going to make much difference, you could hardly even see that measurement! and I'm sure that the quality control people would have trouble keeping it inside that tolerance. ( forget about the third and fourth digits past the decimal point).
Also you could easily get different quality and strength between sheets of balsa which would probably be much more significant, but is usually overlooked, I say go for the stuff you can get easily.