15/45 does not sound right for a switch that hasn't been tampered with. Factory out-of-the-box settings are often printed on the underside of the cover and they normally have a spread of 20 PSI. When a switch starts to go, it is usually the low end that suffers. Sediment or mineral builds up under the diaphragm holding off the start.
The same mineral buildup can cause the pressure gauge to read wrong. I would replace both the switch and the gauge and I would snake through the nipples connecting them to the tank Tee to make sure they are clear.
The choice of pressure range will depend on water usage, the pump, and from what depth it has to draw. Pump output will be on a curve with fewer GPM at higher pressure. If you set the pressure too high, there is a chance the pump will not be able to attain it and keep running. A little experimentation and measurement can ascertain at least at that moment in time what the pump is capable of. With the cover off the switch, taking care not to touch the electrics, you can override the spring temporarily with visegrips to see how high the pressure will climb while at the same time measuring how much you can draw off at the drain cock and not drop the pressure.
If you put an air pressure gauge on the tank when the pump has reached the cut-off, both the air gauge and water gauge should read the same. If they don't, one of the two is reading wrong. After you have the pressure switch dialed in to what you want, you need to turn off the breaker, completely drain the tank, and set the air pressure to about 3 or 5 PSI below the cut-in pressure setting. Make sure both air and water gauge reads the same at both ends of the scale or else compensate for the deviation.