If you want flats, add about 5 to 7 percent to the tension figures for typical flats compared to typical rounds. Again, what you're wanting is relative tension.
Here's what I mean: first, strings by genre are similar in construction, therefore similar in tension. It has to be, or you wouldn't get the similar tone and longetivity of strings that does exist from brand to brand and model to model. All the differences that are discussed on the forum and in "string shootouts" and such are subtle. For these purposes, a round is a round and a flat is a flat.
Now, regular basses are tuned, of course, EADG. Red Mitchell, or 5ths tuning, is CGDA. Therefore, the D string is the same. Find a D string you really like in brand, model, and gauge, and compare it to the D'Addario chart. For example, a D'Addario Chrome flat .060 D string has about 45 pounds of tension. So cross reference the chart and see which string diameter for the given pitch has about the same tension, and go from there. It doesn't matter if the string brand, model and gauge actually has 45 pounds of tension, or 44, or 46, or whatever. What matters is that since the strings will be made in a similar manner, the appropriate gauges for the other strings can be extrapolated generally. Since the strings are .005 apart, and not .001, even if CircleK does .002 or .003 differences, it will never be exact. Close is what we're looking for.
Another way to think about it is to add or subtract about .005 in the gauge per pitch difference. So, if a regular medium or medium light G string is about a .045, then to go with an A, try a .040. And a B string is about a .135, so a C would be about a .130 or .125, if a lighter feel is desired. Then a G would be about a .90 instead of .85, give or take.
Think about it: every guitar manufacturer of nickel plated rounds markets a standard set of 10-13-17-26-36-46, and with subtle differences they are all about the same. It really is similar with bass strings as well.
