I buy food grade glycerin by the gallon from Amazon. Buying in small bottles from a LHBS is prohibitively expensive.
In 2022 I made 76 gallons of wine, averaging 0.84 oz glycerin per gallon. My lowest amount was 0.63 oz and the highest was 1.11 oz. I used the most glycerin in kit/pomace wines (kits fermented on pomace from grape wines) that tasted acidic. A bit of extra glycerin changed the perception.
My method is that we prep a wine for bottling and taste it, reserving a large glass for comparison. I've done this enough that I can ballpark the amount of glycerin to use, so we add it, stir very well, and taste again. In the case of the kit/pomace wines we started with 0.8 oz/gallon, then bumped it up when the first tasting was still noticeably acidic.
As I often say, go gently because it's easier to add more than to take some out. but in this case, if you add an extra oz of glycerin to 23 liters of wine, it's not going to ruin it.