Whenever I'm working from a real floor plan, I mostly go for accuracy. I take the dimensions of the house and divide it by three before rounding up. So a 42'x55' house in real life would be 14x19 in the sims. After that, I put the floor plan in a PaintToolSAI (just because I like it, but GIMP or another program would work well too) and scale it down to a grid I've made fitting in the dimensions I've got with some slight adjusting for uncentered or half tiles. There's just a lot less fiddling with it that way.
Once I get it in game, I might adjust a bit for sim flow or some remodeling.
