I still don't see the 18+5 until you finish row 1 which ends with 41sts (18+18+5).
For the cast on, once you have one repeat plus the begin and end sts (18+18+1), the additional repeats are +18. So you can make this pattern as wide as you'd like. Good luck with it and with the laundry cabinets. Love to see a photo of the finished knitting (and ok, the cabinets too).