Use the "concentration times volume" (C1V1 = C2V2) equation to calculate the dilution rate. For example, if you wanted to make 3 quarts of yeast starter at SG 1.020, and your saved wort is SG 1.058, you would set the equation up this way:
20(3) = 58(X)
Solving for X would give you 1.03. In other words, you would need a just a hair over a quart of wort at SG 1.058 to make 3 quarts of wort at SG 1.020.
You can use any units for concentration and volume, as long as you use the same units on both sides of the equation. If you use specific gravity for concentration, use only the numbers after the decimal point. (As in the example above, SG 1.058 become "58" for the equation.)