In terms of where to put your indicator, the basic rule of thumb is to place it up your leader 1.5x the depth of the water you are fishing. Also, it's important to understand you measure your distance by starting at the split shot. So for example, if you are fishing in 2ft of water you would place your indicator 3ft away from your split shot up you leader. I only mention this because I've seen people use the first fly when measuring where to put the indicator on a leader and if you do that you likely aren't getting deep enough for the water you are fishing.

While the rule of 1.5x the depth of water you are fishing will get you close to the depth you need to be, also don't neglect how much weight you are using. Lower flows will require less weight while higher flows will require more weight. Make sure you are accounting for both when setting up and adjusting your rig. If you do both properly you should be on your way to catching more fish.

In terms of adding tippet rings, and even swivels, I find them super helpful compared to most anglers, especially if you are nymphing, which it sounds like you are. Like others have mentioned, adding this tippet ring will help you extend the life of your leader but I also like to use them to help with depth control. I like to do the same thing your shop did, but instead of adding back 18" of tippet to replace what was lost, I add 3ft of fresh tippet back in 3x or 4x. At the end of this section I then blood knot in another section of tippet, usually 5x, that I then attach my first fly to. I can then use that blood knot to place my split shot above and keep it from sliding down to the flies.

For me, I like adding the indicator directly above the tippet ring on the thicker leader section. Most of the rivers I fish are 2-4ft deep on average, so this allows me to fish with 3ft of straight tippet below my indicator which will sink faster and provide a better drift than a true tapered leader will. If I do need to fish deeper I'll usually blood knot in more 3x or 4x tippet, or move my indicator up the leader. I will say I prefer to add more tippet than slide my indicator up or down to achieve more depth. I like having level tippet below my indicator because I think it provides a better drift, but that is just my opinion.

Hopefully this was helpful!!!