I deleted a post because i'm not feeling confident about the suggestion
Another way is to figure it out as you suggested, working from your favourite B A D G strings ...
C: I understand there isn't much gauge choice for flatwound B strings, so a B string tuned up to C ... Bs are usually loose so this improves the tone.
D: The D string is unchanged so your favourite gauge for that.
I used 'long scale Chromes' tension chart here
http://www.daddario.com/upload/tension_chart_13934.pdf to figure out that a drop in gauge of .005 compensates for a standard G tuned up to A. Similarly an increase in gauge of .010 compensates for a standard A detuned to G.
G: Your favourite gauge for standard A + .010
A: Your favourite gauge for standard G - .005
Example DR legend flats medium BEADG 125 105 85 65 45 -> CGDA 125 95 65 40
But my preference and recommendation is an equal tension set to make fifths more comfortable to play, since there will be more stretching and shifting. For a medium-light/light example: 132 85 55 35.