As to the add-on target turrets like that sold by Stoney Point, IMO, not worth it.
I had purchased a set for a Leupold M8-4X on my Browning Buckmark Unlimited. Unless you scope is at (or close to) the middle of its total adjustment range, you might have some problems. Where your crosshairs are within the adjustment range of the scope depends on how far off in alignment your rings are with respect to the bore and the distance of your zero. The setup of my scope/gun was such that when zeroed at 50 yards, the elevation was towards one end of the elevation range.
Now the Stoney Point turrets have a limit to thier adjustments also.
When you make adjustments to your scope, you are essentially physically screwing up/down//left/right the crosshair (or more accurately, the section of the scope that contains the crosshair). The screw physically goes in and out of its threaded hole in the tower. When you screw on the SP turret, internally it has a limit to the amount of movement the piece that engages the slots of your scope's adjustment screw also. It does not have the same range your scope has internally. This is due to the fact that the cover that replaces your regular adjustment caps is fixed. If you are close to one end of your scope's adjustable range, the engaging piece can either not engage (if the slot of the screw is too far) or bind up (if the slot is too close) when you try to zero the scope. (I had the problem of binding up. Had a heck of a time getting it unbound too.)
In short, the SP type turrets limit your adjustable range and may not work if you scope's crosshair is not close to its physical center with respect to the scope's main tube.