You should practice estimating your exposure just in case your meter dies (or you can't find it, like when Ansel was getting ready to take his famous photo, Moonrise Hernandez.) See our video: Top Ten Things That I wish I had known when starting in Large Format Photography,
Most of us have heard of the "Sunny Sixteen" rule but what do you do when it's not sunny? The standard procedure is to estimate the EV based on the lighting conditions. You then look up the proper lens setting in an "EV table" that gives you every possible combination of shutter speed and f/stop.
There are a couple of problems with this approach. First, you need two charts: one to help estimate the EV and a second one to lookup the actual lens setting. Second, the EV is only valid at one ISO setting (normally 100), so if you're shooting at ISO 200, you have another step to remember. Third, we'll add a third objection, most charts, both for EV and lens settings, expend a lot of space on unrealistic situations. I'm referring to conditions so dark that reciprocity is a major concern or to light conditions that never occur in nature.
So we created our own using data from various charts, along with some of our own measurements. We also, say we say, optimized it for large format.
First, we combined it into one chart. (Frankly, the EV number isn't all that useful but we included it for reference.) We then provide a shutter speed/ f stop combination for different ISO ratings appropriate for large format. Obviously, you can adjust the setting to suit your particular needs.
Now if you're shooting at some other ISO, you'll have to interpolate between values. But you're just guessing anyway, so don't get carried away trying to be overly precise.
Hint: if you're not sure, lean toward overexposure. This is true for most films but especially true if you're using Foma film (known for its wide exposure latitude) and SP-76EC (which has an inhibiting agent to help prevent over development).
But, with a little practice, you should get close. After all, it worked for Ansel.
Click the photo to download a pdf version (it includes our Shutter Speed Compensation Chart as well.)
By the way, the Shutter Speed Compensation Chart and the Exposure Guide are on the back cover of our Photographer's Logbook.