First, see if you get 15 sts/4", you might be tighter or looser. Hats are generally knit about 1-2" less than the head size because they're stretchy, so you might aim for 18" or a little more. If you do get the same gauge 75 sts would be 20" and way too large. You want an even number of sts - divisible by 2 for 1x1 rib, or 4 for 2x2 rib. Knit the ribbing for a couple inches, then switch to stockinette. The ribbing should be stretchy enough that you don't need to increase sts for the hat part.