I have another ruler in the scale that I'm building the model in. That way, whatever the dimension is on the plans, it is the same on my working stock. I find it easier to work this way, I guess because I've always worked from general arrangement plans from shipyards. No, I don't have a lot rulers around. Building superstructures from acrylic plastic I've been working on faster and cheaper ways to scratch build model boats.
Most clear plastic comes with a film on it to protect it from scratching.
It's important to leave this on till ready to glue structures together. I usually mask off the plastic over top the film, but sometimes the film keeps coming off. In that case I remove the film and put masking tape on both sides of the plastic to avoid scratches. Then I cut out the walls for the cabin superstructure. I know which wall goes where.