What I have done is overlap the workbench radius to build my settlement. For the walls I built all the walls inside the radius of my outermost workbench. I haven't had a problem with mob spawns in my base and when the raids happen all mobs spawn outside. A thing you will have to watch for is the deterioration of workbenches that are not under a roof (deterioration can be disabled with mods) and monsters targeting your workbenches. Any spot not covered by workbench radius (speaking solely from experience) can be a spawn point for mobs.