You're probably better off just taking a test shot to begin with then looking at the brightness histogram on the shot you just took to see if you need any adjustments. Generally, you'll want a nice distribution where the graph looks like a bell curve or has a balanced range. This tells you that you have a good distribution of tones, meaning you capture the nuances of the landscape. If you see the graph skewed so much of the weight is one the left or right sides, generally your photo will be under/over exposed depending on which way the graph is skewed.
It looks like the lightmeter and your camera have about a two stop difference, i.e. SS of 1/160th vs 1/640th of a second. There is no "right" answer, but choosing the settings that make the picture more pleasing to you/client/etc. For example, if you took a photo using these settings
F: `5.6` | ISO: `400` | SS: `160`
and another using these settings
F: `5.6` | ISO: `400` | SS: `200`
I'm pretty sure no one would notice any difference unless you literally had them side by side. So, personally, I would say the answer to your question, whether to trust the camera or formulas, is both and neither. Use them as a starting point, and adjust as necessary. Also, assuming you shoot in raw, or even if you don't, you can always make adjustments to the exposure via post-processing. Obviously, you can't work magic, but as long as you're pretty close it doesn't matter too much in the end.