I agree with Sharon, you should be able to feel the ribs, and also the bones around the chest.
There is a lot of variation in Shelties, not just in height but also in length, so an average is hard to determine. But have a look at this chart with heights and weights of some of the Shelties we've had on the board
https://docs.google.com/spreadsheet...DNrvWtvu-PUyUSt2Aa0/edit?hl=en&hl=en&au#gid=0 To get height you measure from ground to shoulder blade.
My Mr D, a very, very long Sheltie, was over 16 inches and hovered around 23.5-25.5lb for most of his life. A 45lb Sheltie would have to be Aussie Shepherd sized - which is entirely possible, Shelties can be way over sized.
Remember Shelties were bred by poor farmers who valued a dog who could scavenge for it's food. So we ended up with a breed that is a) extremely fuel efficient, and b) darn good at scavenging.
A few other suggestions. Dogs like routine - they like set meal times. You can add in extra meals as you don't have to feed just once or twice a day, but they should be set times. My dogs get half an egg before bed (to hide tablets) and love it. Giving ad hoc treats during the day will encourage looking for food outside meal times. Dogs actually love working for their food so think of using a treat dispenser or hiding the food (my dogs love this one) or scattering in yard. Sometimes scavenging is just boredom. You can make an easy dispenser by cutting holes in a water bottle they push around to get the kibble out.