Conversion

Hi,

I'm working on a DRO and am having some confusion over the conversion of
metric units to inches and back again. The device should be able to read
glass scales down to 0.001mm or 1 micron so it is important to get this
right.

I have always used 1" = 25.4 mm but on trying to work it out with a little
more accuracy I find that an English mile = 1.609 kilometres or 1" =
25.39457071 mm. but this figure changes depending on who it is coming from.

Does the list have any opinions on this.

Regards,

Terry