My current scheme is to mix in RWC with my soil mix at 10-20% of the volume, add a nice layer on top of the soil at pot-up, and then give the pots additional castings once a week at a rate of 2 tsp per gallon of pot size in veg, and 2 tbl per gallon of pot size in flower, as well as malted barley at half those amounts. I also have some worms in the pots to produce some fresh stuff daily as well.
Plants seem happy enough. I really don't think you can overdo it though. I used to add an inch or more to the top before mulching in and never had a problem. The covering mulch is important as I stated because if they get wet in a clump and then dry, you're working with a concrete like material and nobody wants that.
I may be wasting castings as much of the commercial suggested rates are something like a handful at pot-up and then maybe another top-up a few months later, but I harvest more castings every 8 weeks and they have to go somewhere.
Castings are supposed to be a time release type of amendment so I figure by adding a little bit more every week I always have some in the sweet zone, and more that will be in the sweet zone as time moves on.
I did a Jadam extract of them in a little clear container so I could watch it breakdown, and the water darkened over I'd say a 1-2 month period as the castings broke down and released their water soluble goodies, so the commercial guys are probably right on track with their recommendations.
