You don't really have to guess how many stitches to put between increase stitches. If you need to add 12 stitches in your increase row, divide the number of stitches in your ribbing by 13. The result tells you how many stitches to knit before each increase, and after the last one. Just add a 1 to the number of stitches you're supposed to increase, and divide into the number you're starting with (the ribbing). If it doesn't come out in round numbers you have to round up or down and adjust at the ends of the row.
Say I start with 96 ribbing stitches and need to increase 15 stitches. I divide 96 by 16 (15 increases + 1) and I get 6, so I would knit 6 and increase on the 6th stitch 15 times, and I should have 6 stitches at the end which I knit even. If I started with 96 and needed to add 12 stitches, I'd divide 96 by 13 and get 7.38. I'd round that down to 7, multiply 7 x 12 = 84. Subtract 84 from 96 = 12, to be divided over the 2 ends, so I'd knit 6 at the start of the row, increase on the 6th stitch, then do my next and subsequent increases on the 7th stitch, and should end with 6 at the end of the row.