Designing a telescope?

I would like to design a telescope from the ground up, the only parts that I will not be making myself will be the mirrors and eyepieces. I've already decided I needed focal length equations, what others will I need to build it? I'm gonna go for a truss tube dobsonian, (I've seen some ideas for ultra lightweights that I'd like to play with) which means I'll need some other equations I'm not familiar with. I've not decided on aperture size yet, I guess that will depend on what I can afford mirror wise. So any ideas? I figured here would be the best place to ask.