When building dioramas I like to cut all my 2”x4” wall studs, 2”x6” headers, sill plates, top plates, rafter trusses, etc. at the same time, its less time consuming & just easier. You may have to cut a few pieces later but the bulk of the cuts can all be done at the same time. Once you figure what size lengths your 2”x4” wall studs are, cut one & then use that one to cut all the others so they are all exactly the same length.
Cut the roof trusses, once you figure your pitch (you know what it is because you cut your end wall for the building). To get the angles right take a piece of wood that you are going to use for the tops of the roof trusses & hold it up & down or vertical against the end wall of your building this will give you the proper angle for this cut. Mark your angle & cut it then take that piece & hold it against your hardboard roof panel for the proper length, make sure to cut that bottom angle the same as the top angle or it will not look right.
Now lay the two pieces against your end wall so you can figure what you need for the two support boards to hold the roof trusses together. Remember to figure out where your 2”x4” top plate for your studded wall will end up so you know where the bottom roof truss support should be located. Now you know where it will sit & the length you need to cut it, the top support board can be placed where you like it astatically its only function will be to help keep the truss together & for looks.
Just like when a real house is built they have a stack of 2”x4”s, 2”x6”s, roof trusses, etc. ready to go when the framing begins.