If using a flashmeter (which is always an incident reading type) ie. Sekonic L758D, the ambient/flash balance will be worked out for you based on your selected aperture together with the piddly 1/30 sync of the Pentax 67 / 6x7. A hand-held meter will also provide variation in the balance of ambient/flash in percentage. Sekonic, Minolta AutoMeters, Polaris... there are others, all have their strengths and vices and all can be readily had for good savings on the second-hand market. I bought my L758 for $1080, but it can be obtained now for $400 or so.
The use of a flash meter is strongly preferable for analysis of ambient/flash to getting bogged down in several levels of figures. It's a simple and stress-free way, but if you're the scientific type, by all means let rip with LVs, EVs, ISOs and whatnot, but your prime consideration should be just to get a picture and inspect the results, not argue about variables. If the first pic doesn't work out, then keep trying. Experience is a great teacher.
BTW, I would strongly angle for a leaf shutter lens for the 67 (there are two available, but only one still made by Pentax) to enable higher sync speeds and thus greatly more flexibility, but it will depend on what you are photographing and in what conditions.