I have no idea whether there's an accepted standard, it's more of a personal preference I would think.
I just keep my basement man-cave between 45-50 % mainly because it's loaded with expensive guitars.
In the Winter it gets pretty dry with a forced air furnace running, so I run a small portable sunbeam warm mist humidifier.
Unfinished Basement Theater & Whisky Guitar Lounge
JVC-DLA X750R | 120" Screen | LG 65" OLED 65B6P| Panasonic DMP-UB900 | HDFury Integral
Yamaha RX-A3050 + RX-V3900 | Atmos 7.4.4 | Mission M3i Speakers | Funk Audio 18.0 sub's