Tolerances should not be held to significant figures, they should be held within the accuracy bounds which are needed. If you need three digit accuracy, and put only two digits, then the machinist will give you a part with two digit accuracy. Conversely, if you give him (or her) four digits then more time would be spent trying to hit the four digit accuracy within the given tolerances.
Back to the point. Give tolerances as needed, do not add an extra decimal because of mathematical operations.
Taking the science out of rocket science...One step at a time.
In the typical American (mechanical drawings) tolerancing system the number of decimal places corresponds to tolerance range given in the title block.
For instance 3dp may be +- .005, 2 dp may be +- .01 and 1 dp may be +- .03.
In other countries/working to other standards general tolerances like this are done differently.
When doing tolerance calculations I'll usually use all the decimals available and then round the result as required at the end based on function/title block tolerances.
KENAT, probably the least qualified checker you'll ever meet...
