I don't recommend them for the back pocket (a flat wood pencil from Home Depot might survive), but my all time favorite mechanical pencils for in the wood shop, layout for chip carving, at the lathe, the bandsaw, and when designing things on paper is the Pentel GraphGear 500. I keep all sizes (.3, .5, .7 and .9mm), one of which should meet your "reasonably fine" requirement, whatever that means.
The body color indicates the lead diameter. The clip is steel so it sticks to magnets I mount above the lathe, on the bandsaw, etc.
For the back pocket, a retractable would be better but I don't have a favorite. Years ago I turned some pen/pencil sets from kit hardware and one of those retracts the lead quickly with a twist.
0.5mm pencils - the brushed steel barrel zebra type - though they wouldn't have tolerated sitting on them. Always liked them because they go out to a very fine point (the end of the barrel has a gradual taper on it instead of being relatively blunt like some of the old lead holders were).
For heavy marking in the shop, any of the premium pencils that are a little soft, especially when they get short from writing - it feels like you get your money's worth if you write with them when they're long and use them to mark wood when they're short.
Their virtue is they leave a dark line easily and the lead is a lot more break resistant than common pencils.
However, I would not recommend them for back pockets, as with any precision measuring instrument. I suffer from sciatica, so I don't keep anything in my back pockets. You can get them individually, as well. They are also available in a set of four with the 5 mm replaced with a 4 mm. I found these based on a recommendation by Chris Schwarz.
For many marking tasks, I still use a 5 mm Pentel Pilot P500 gel pen. They are especially useful for marking dark woods like walnut. The tips are fragile, so I limit their use to surfaced stock. The gel ink doesn't soak any deeper into the wood than soft pencils, so the marks are easy to erase with a smoother plane or fine sandpaper. Thanks to Bill Tindall for finding and recommending these, but it appears he has upgraded to newer technology.
In all my years of using mechanical drafting pencils I've never once tried a .4mm so I had to order one. Sometimes .3 is too fragile and .5 is too thick.