You figure out what the multiple is. You're lucky that 56 is a multiple of 8 times 7, so for the decs you can either do k5, k2tog 8 times or k6, k2tog 7 times.
If you had say, 66 sts, thats 11 times 6, 64 is 8 times 8. If you end up with a weird number like 83 you could add a stitch to make it 12 times 7 - or just work the first dec round as k9, k2tog, then k10, k2tog 7 times.
sue- knitting heretic