What pressure should I aim for when setting a pressure reducing valve?
Assuming metal pipe, I'd set it to about 60, because that's a good compromise between too-high pressure and disappointingly little pressure. That's what most well pumps I've seen are set for, and significantly less than most municipal water systems (I've seen one city water system that delivered 125psi to the house). Much lower than 60psi and you'll be frustrated by the slow flow.

With resilient (not PVC/CPVC) plastic pipe, I'd go a little lower than that because unlike metal pipe, which has very strong, secure connections, plastic pipe has only clamp-over-plastic-over-barb connections, which can fail more easily than metal pipe. With PVC/CPVC, I'd go even lower because it gets brittle with age.