This is for anyone out there who would like to know a metric to imperial conversion trick on a calculator. I only post this due to another thread where there was a lot of mixed measurements due to measuring device availability.
On a calculator with a tax- and tax+ button, set the tax as 2440. When you have a measurement in inches, hit the tax+ for millimeters, and tax- for millimeters to inches.
We use this at work dealing with European hardware that are metric and the cabinets we make using imperial measurements.