I got the CYA iwb holster for the shield.
Upside down, shield doesn't fall but if I jerk it with its weight, it falls off.
How much retention is good?
Thanks

My usual test is to use a few light shakes while the gun/holster is upside down. If it doesn't come out, but I can draw it properly while wearing it, that's good enough retention for me. The key is balancing retention with ease of draw. If the gun doesn't come out during the shake test, but drawing it is difficult, that's too much retention.

FWIW, my Kramer IWB #3 holster will not release my PX4 Compact with vigorous shaking, but drawing it is easy, so it is possible to get a good balance.

Ok thanks.
I'll try it out once the belt comes in.

Good idea holding off until you have your belt. That may make a difference especially if you are using a leather holster. When adjusting, make sure you leave the holster in your pants and your gun in your hand. In my younger days I would holster an UNLOADED gun and roll around on the ground including a couple of somersaults.
Now being much older, I make sure I can draw without injuring a rotator cuff.

Best off to set retention by wearing the holster and pistol, either unloaded or loaded with dummy rounds, and go through whatever physical things you expect to encounter. You don't want the pistol to inadvertently fall out of or become dislodged from the holster, but you also don't want so much retention that you cannot easily draw the pistol from the holster.

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VCDL, GOA, NRA