Relationship of strings to one another

Greetings., all.

My dulcimer currently has .011, .011, .013, and .024 strings. The strings work well together for a sweet sound, but I would like to lower the pitch a little. I'm thinking my dulcimer sounds like a soprano, and I would like it to sound  alto or tenor. Not bass, mind you. 

So my question is, is there some mathematical  relationship/ratio within the strings I have to follow when I replace my current strings with thicker ones? Can I use .013., .013,, .015, and leave the last one alone at .024 or would I  need to replace the .024 with a .035? (Sounds a little thick to me.) 

I went to the Stromberg (?) site that calculates string gauges based on VSL, and it recommended even thinner (higher pitch) strings than I have now.

Hmmm .... whattodo, whattodo ....