I have a Sekonic meter that does reflected and incident readings. I use it with all of my cameras, metered or not (I prefer the incident reading). I have shot 400iso film (usually Tri-X) in my old meterless Pentax so much I can guess accurately enough for most situations.
If I am lucky enough to own a meterless Leica I would most of the time use it without a meter. At times I would bring either my Minolta flashmeter III or flashmeter VI. I would never buy that meter attachment from Leica to put it on the shoe.
I don't have a Leica but a meterless Mamiya RB67 and use a Minolta IIIf Autometer. It provides reflective, incident and flash readings and comes with a 10 degree spot. You can also get a 5 degree. I tried the app on my Samsung S4, but it wasn't always accurate.
My "force within" is not so forceful and needs a tune-up so I rely on the meter. Also, the force within does not work well if you're shooting during "magic hour", one hour after sunrise and one hour before sunset, when the light changes rapidly. Best time to shoot landscapes. Could be changing in stops every few minutes. Guessing at this point is really just taking chances on the result. Even with the meter, I bracket. And I'll tell you that sometime I'm glad I did. I also usually shoot chromes which have less room for error than negatives.
So many well known photographers never used a meter so it's very possible to live without one.
Sunny 16 is simple and consistant.
The general rule is ISO/16 so for ISO 400 it's 400/16, 100=100/16 etc. The exposure never changes for bright sun.
Well almost never. Sand or snow & ya gotta stop down another ~2stops. Dark subject & open ~a stop or two.
Open shade? add 1.5-2 stops.
Cloudy bright, no shadows, +1.5-2stops.
Heavy overcast, +3 stops.
There's a learning curve but thats true even when you use a meter.
I use a Luna Pro F or Spectra 500. Mostly for playing with pinhole.
Don't need no steenking meter for Leica stuff.