I've had some issues with upsizing LPR rockets where the fins turn out HUGE. is this the case here? usually lpr birds have oversize fins t start with simply to ensure stability. for a scale model I would check to see what the actual fin size is and scale that down to match the model's scale. for a non-scale rocket such as the Estes Goblin, I would scale the rocket up to whatever tube size I wanted (a 3" dia. tube is 234% upscale) then scale the fins down to 75 - 90% of the upscale version(so they are not as large).
For upscale/downscale brainstorms, I usually make a list of the relevant dimensions in excel and multiply the column by the scale factor.
I have heard of OpenRocket's scale feature, but I've never remembered to use it.
If you generate your nosecone, any body tube/couplers will be scaled to its diameter. Same for tubes after a transition. When you choose your motor mount size, centering rings will automatically conform to the space between airframe and motor tube. That just leaves the fins.
Scale Factor is needed to determine material and part size differences. It's no more then a simple math problem.
If you have a model based on say BT-20 (.736"OD) and you want to Up-scale it to BT-60 (1.637" OD) divide the Up-scale diameter by the base model diameter to get the Scale Factor in this case 1.637 - .736 = 2.224 scale factor. You can now Up-Scale all the remaining dimensions of the model (except Angles which remain the same) to build you larger creation. example: Base model Fin Root edge @ 1.5" x 2.224 = Upscale fin Root @ 3.336"
To Down-Scale the same BT-20 (.736") model to say T3 (.375") body, you'll use the same process to find the scale factor .736 - .375 = 1.963 Scale factor. but you'll reverse the process to find the multiplier .375 div by .736 = .5095108. Base model fin root 1.5" x .5095108 = .7643" Down-Scale Fin Root.
Computers are great but we should also know how to work the problems manually. Helps to keep the grey matter a little sharper