Well, I truly gave myself brain ache this evening! 8 years on from GCSE maths and I had to do a bit of algebra....all to enable me to start on the knitted shrug to go with my polka dot dress.
There is probably a really easy method of doing what I did but I really couldn't think of a more logical method - it's late ok!
Basically the wool recommended 6mm needles and said that it would make up a 10cm x 10cm square by using 14 stitches and 20 rows. I want a chunkier knit than that so I'm using 10mm needles. Since I am making up a pattern in my head ( I can't follow normal patterns so I look at something I want to knit and just break it down and work it out like that! I'll get the hang of patterns eventually.) I needed to know roughly how many stitches to cast on. Cue brain ache! I had to work out how many of the stitches from a 10mm needle would make 10cm in length.
So i started off with - 100 stitches 6mm = 60 stitches 10mm.
So far so good...
140st x 6mm = 100cm.
Which is 1.4 st/cm.
We'll call 6mm st A and 10mm st B.
So (100 A) = (60 B) and (140 A) = ???
I got there with it though and concluded that (140 A) = (84 B)
Easy from there as 84 B = 100cm, 8.4 st = 10cm and I wanted to have roughly 46 cm so that was 39 st, plus a little extra for a sort of seam allowance.
Hallelujah! I cast on 45 stitches, did a few rows, measured 10cm and it was pretty much halfway between stitches 8 and 9!
So after all that effort I hope it turns out ok! Will post up when it's done :)