Yarn conversion yardage question

Is there some formula you can use to calculate yarn conversion when switching weights? Say an item calls for 1000 yards of WW yarn and you want to switch it to DK, but make it the same size. I realize a gauge swatch is in order and all, but I’m really baffled by this, especially something that has a pattern in it… would you really just try out some needles, pick the one you think looks the best, and then add on the necessary amount of stitches to make it the same size? like say the recommended needle size for the WW is 7 for a gauge of 4 st/inch and then the recommended size for the DK is 3-4 for 6 stitches per inch. If you altered the needle size on the DK to get 4 stitches per inch, you’re going to have a much holier <?> fabric, so i would assume you would go with a 4 (or even a 5 in this case) to get the appropriate look, so you might still wind up with 6 stitches per inch, and then you’d have to add 2 stitches onto the CO for each inch of the end measurement, yes? so if that’s all right, or close to right, where does the yarn requirement come in? i’m looking at some ann norling blanket patterns, and it seems there’s a trend between 4 st/inch and 6 st/inch of about 30% more required for the smaller gauge. is that safe to go with?

thanks bunches!