Sunny 16 is an alternative to metering. That said, I hope nobody tried to convince you it was better or simple. You do have to understand that the meter does lie to you until you understand more about how it works.
Sunny 16 says on a bright day, set the camera to f16 on the aperture and set the shutter speed to a reciprocal of the the film speed.
If the film speed is 100 (don't confuse this with whether the film speed is stated in ASA or ISO.. they are the same), then the shutter speed should be the closest you can get to 100.
Hence... sunny day, film speed 100, aperture f16, shutter set to 1/125th. ... proper exposure for an average scene. You are shooting for the light falling on the scene.
Hence...change this.. sunny day, film speed
400, aperture f16, shutter set to the closest number to 400, ie 1/500th.
In your example, you set the reciprocal for 400 to 1/250th. I don't know what you mean by thin negative, but I would not have used 1/250th as a reciprocal for the speed of 400. Also consider that the adjustment for sunny day IS affected by the differences in the nature of the light for early morning sunny, high noon sunny or late day sunny. It's more a matter of using "sunny 16" until you develop your own interpretation of how the camera/film combination works for you.
One of the things that film photographers used to do was work with one camera and only one emulsion of the same brand of film until they knew it's response to the light.
and so forth.
But you do have to spend a lot of time understanding other variables, and how the meter works. It does not do what you think it does. Modern computer driven camera's are using incredible programming to try and simplify how meters work, but in the final analysis, I always have this thought in my mind.
No matter what I interpret in my mind, and no matter how much computerization is built into my camera, you still only get one shutter speed, one aperture and one ISO/ASA with each click of the shutter. That's why we used to bracket exposures... as measured, then one over, then one under and gamble on one being correct.
Film does pin you down on the film speed because once that film is loaded you only get to shoot that film's speed until the roll is finished. So you do get to stay with only two variables to each image.
I am going to link you here to a page that is very pointed about all the variables, and discussion about "how the meter lies".
The K1000 was the chosen king of student camera's in its day. It's a great manual camera, and it does have a meter that does not use programming to set any of the settings. It simply suggests a starting combination of aperture and shutter speed, and as you change either one of the aperture or shutter speed, it tells you how much you are varying from it's suggestion. However you are in control. You learn how the settings you choose can run up and down the scale of aperture/shutter speed combinations.
This link is a little intimidating, but it's loaded with a ton of usable understanding of manual control, and changes in scene and lighting. Have fun:
http://www.fredparker.com/ultexp1.htm