Here's a clarification...
First let's separate the set-up move from the sweep, d-loop and cast. You've already done that in your post so we are on the same page.
Now imagine a double spey from the right bank, downriver wind. Set-up; sweep to D-loop, and cast. Beautiful.
Now imagine again a snap/circle from the left bank, upriver wind. Set-up differs, but the sweep to D-loop and cast should employ the same characteristics as same on the right bank double.
Perhaps this will be clearer...
Let's go to a very shallow round lake and walk out to the middle. No left or right bank, no current.
Now do a double. Then sweep to a d-loop, cast. Gor-geous.
Turn to face 90 degrees to the top hand side and then do a snap/circle.
The sweep, d-loop and cast should be nearly, if not identical to that of the first double. In my case, they are identical by design which gives me total consistency with both casts.
Many of my students make a different motion after the setup of a double vs. a snap or circle when in fact the moves employ the same mechanics. Most often someone with a great double will hurry a snap/circle or shortcut across to the d-loop instead of sweeping as they just had with their double.
My point is, you know one, you know the other as they are (aside from the setup) the same cast.
Hope that was clearer.
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