The original pattern uses Rowan Cashcotton, (35% Cotton 25% Polyamide 18% Angora
13% Viscose 9% cashmere ) size US 1 or 2 needles, 7 spi with 8 garter ridges/inch,
and 35 sts make a bootee that is approx. 3 ¾ inches long.
I am using Sirdar Snuggly yarn which is nylon and acrylic with US 4 needles. My knitted a swatch which gave me approx. results of: 6 spi , 6 garter ridges/inch, cast on 24 sts which measured almost 4 inches.
I need the bootees to fit about a 5 inch foot and am not sure about the gauge,
adapting the pattern, inc. and dec. etc to fit the larger size.
I am not able to figure the math out and as these are needed soon I don’t want to guess on it either.
It is easy to read that many of you have been knitting for years and can figure this out easily.
I think you calculated the wrong direction - with thicker yarn and larger needles you need to cast on less than the original 35 sts, not more. I came up with 29 when I calculated the gauge percentage difference, and I’m not sure that’s a good way to figure it.
Duh, I just had an idea… In the original gauge 35 sts = 5". In the gauge the OP is getting 5" would be 30 sts. So I guess the 29 sts is the right way to go.