I bought a gauge and notice if I set the dehumidifier on 40 it goes below 35 before it stops. If set at 45 it goes up to 50 before it starts and stops on 45.  Which would be best for guns ? Some have wood stocks some don't.

45% - 55% at around 70 degrees is just about perfect.

Tossing humidity gages in trash. Have three gages and did the salt & water test. One read right on other two I needed to add 2 and 7 degrees to what it read. After a day they all read something different. After adding the 2 and 7 degrees none match the one that read correctly. I decided to just set the dehumidifier on 45 and forget it. Does that sound good?

Yes

I've had the same experience with every hydrometer I've tried.  None seem accurate.  If you can calibrate them, the calibration only lasts a week or two.  I've found them worthless in my humidors and gun safes.....

ETA:  I don't think wood stocks cracking is a big issue.  I would imagine we'd have reports from gun owners all over Nevada/Utah/Arizona talking about their cracked stocks or having to add humidity if it was an issue.  My opinion, if would be pretty hard/impossible to get the humidity TOO low (but at same time don't need to get it to 0)