Nice to hear from someone from New Zealand. I’ve always wanted to visit NZ, I understand it’s a beautful country.
First, thanks for buying my shed book; I’m glad you liked it. Now, regarding roof rafters, the first thing you need to know is the pitch of the roof and the length of the rafter. Since you’re dealing with a small building, and are a novice carpenter, I’d suggest drawing a full-scale layout of the roof onto a garage floor, driveway or other large, flat space. You don’t need to draw the entire building, just the top of one wall, the center line of the building and the height of the roof. That’ll allow you to determine the roof pitch (or angle) and the length from the building center to the top of the wall.
Once you know the pitch and size of one rafter, you can cut it and then use it as a template for marking the others.
Sorry I couldn’t be more specific, but hopefully you’ll be able to use that well-known New Zealander intelligence and ingenuity to work out the details. Good luck!–JT