I also have one and it's by the doorway.
I have 3 in the shop and a 5 gallon water bucket. The other thing we should think about is exits and secondary exits. I have a seacan on each side as part of the shop walls and I have thought about...what if I am on the mill and a fire breaks out in a vehicle in the main shop floor? The seacan doors lock from the outside, and unless I change that setup I am trapped on the wrong side of a fire. As my lungs are compromised already, I would not last long in smoke.
So the machine shop doors got changed to lock from the inside.
The other thing I think about is this:
I know in industry and home use, they always advise us to have the extinguisher just inside the door. But if you have ever been in a shop fire, (I have) you will find that opening that door and feeding oxygen to a fire can cause a backflash or explosion, and you still may not get to the extinguisher. I think it is better to have it just outside the door. But that raises the problem of theft and weather damage. I have a 20 lb Ansul with a CO2 cartridge, designed for weather and cold, and it is on the JD 544B loader parked near the door. Ansuls are not at all cheap, but neither is your life.
Just some random Sunday thought...