how do I choose my unit?

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how do I choose my unit?

nerodog
The preferences have a lot of choices for unit.  How do I select that I am in the United States?  My finest measuring tool is a standard micrometer, that is, it measures in thousandths of an inch on a dial that goes around once for every 10th of an inch and has 100 markings around the dial.  Another problem I face is I can't get rulers that measure in 10ths 100ths or 1000ths of an inch.  If metric tools could be purchased, I just might convert the whole thing to metric, but I just spent 3 days looking for an 11mm drill bit, and, not being able to find one, I'll be returning the metric parts that got sent to me, and I'll be looking for American made machines for my uke, since there's no way to create the right hole for the metric ones, and no way to measure a hole I reamed by hand, except in 32nds of an inch.  The lack of tools is really frustrating.  It's easy to convert from inches to milimeters, since there is an exact metric equivalent to everything in inches, because there's exactly 25.4mm per inch.  Do it the other way around, and you're lost.  There is no English measurement anywhere near 11mm, which is 433 mils, so no drill to buy to let you drill a hole that size.  Drills come in even 64ths of an inch at the smallest, with wider and wider spacing between the bits as you get bigger.  There of course is no 27.7/64ths size.  The nearest size would be 25/64ths, and you won't get that piece to go in, or 7/16ths, which would rattle around in the too-big hole.  Luthiers actually drill bigger and shim the holes.  Argg!  Also, people in the UK have started to use the word MIL, which means 1/1000 of an inch in American English, for MILLIMETER, which in American English is a word we don't know without looking for a dictionary.  Likewise, a micrometer is a tool which measures small things (in thousandths of an inch), not a unit of measurement.  The unit of measurement is called a micron to avoid the ambiguity.  Until i started to do carpentry, I'd never used the English system for anything, as I grew up going to private schools that taught in the metric system, and in 7th grade the wood shop teacher lectured me for days for not knowing how big an inch was.  I am truly in Engineering hell.
Can anyone tell me how to set up LibreCAD so I'm measuring in 1/1000 of an inch?  Obviously tolerances larger than that would leave my instrument, which will have sides of 90/1000" thickness, more or less standard, unable to be specified.  The last thing on earth I'd want is to be stuck on a one-inch grid.  As far as I know, no one uses smaller tolerances for things made out of wood.  Unfortunately, carpentry is practiced in this country to the nearest half inch, and people just throw trim on it if the house hangs over its foundations by half an inch.

Sorry for venting, but I'm just so demoralised trying to get started with this software, and it's really frustrating to have to convert constantly.  I wish there were an "I''m a stupid American" button I could press and all the metric would go away.

Nerodog