Personally, I almost never use split shot or a Worm for what it's worth.
To each his own.
I would rather add the weight at the bench.
Stones live in the rivers for 2 to 3 years as nymphs, so you can never go wrong having a big heavy Stone on the line.
Split shot also takes away from the "Natural" drift you are looking for.
You are much better off using your Worm or equivalent.
People say you want to be hanging up on the bottom, and this is true to a degree.
You don't necessarily have to be getting "Hung Up" to be on the bottom. Hooks like the San Juan Worm tend to really get Hung up or snagged, where other flies you might try to get "Down" won't get snagged as often.
With a fly like a heavy Stone Fly, you won't get snagged up as often, you will be on the bottom like desired and more importantly, you will better see the "Actual" strikes.
If you want to use the "Worm", try tying red Marabou out the head and tail. It will give you more action, plus it will allow the current to manipulate it more to keep it from dredging the bottom, when all you want to do is Be on the bottom.
Also, you'd be surprised at how much deeper you will get if you use a Tungston Bead even on a size 16 fly.
Three of these on a line and the big weighted fly isn't needed anymore.
One last tip would be to either use a sinking leader with a floating fly line, or just use a straight run of mono from the Fly line rather than a tapered leader.
Remember, the lighter the line, the faster it will sink.