Just so I'm clear. Get her front feet on a perch, then place a second perch/book in the area that her back feet will hit while circling?
Personally- I would not do it this way. This way has way too much human involvement for my tastes. The dog isn't particularly learning anything other than that there happens to be something in the way of their feet while circling the perch.
Instead, I would simply free shape it. You said that your dog has a lot of value for the perch. This should mean that your dog proactively will run to the perch and stand on it when the dog sees it. If this is the case, then the dog should already have a mindset of putting its feet on stuff. So put out two books at a proper distance apart so the dog can stand on them, and then just sit there with a clicker and treats. The dog should already start trying to perch with all four on the a book. Don't reward it. When the dog finds that this is not gaining reinforcement like it has in the past, they will likely start getting frustrated and will start moving its feet around trying to get on the single perch from multiple entry directions. I would click/treat for ANY rear foot movement that is close to the back book. And then just shape the feet to get closer and closer to the back book. If you have value for standing on things- this really shouldn't be too tough. The main thing is to know your criteria
and be clear in your reinforcement. For me, the criteria would be ANY rear foot movement towards any of the two books. Remember the front book is there only to give the dog something to do with his front feet while focusing on the back feet. So you don't really care about the front feet as part of the criteria (as long as the front feet aren't also
on the same perch as the rear feet).
Here's a training plan for the handstand that I wrote up for my advanced shaping class I am taking right now.
The handstand is a body awareness trick used to teach the dog weight transfer, rear foot awareness, build core strength and front end muscle development. This trick will not likely be fully trained at the end of the course, as it is too intense for just a single month of training. I want the foundations in place at the end of the month so that the behavior is at least half way there.
For the course, I want Pimg to independently be seeking out objects to prop her back legs on. This may be a stack of books, a stool, or whatever. I do not intend to have her using a vertical wall as a prop for this course. Eventually we will work to that level, but it's not likely that Pimg will gain this skill that fast. Other criteria would be a proper nose touch while her rear legs are perched on the stacked object.
Build value for perching with front legs only a block
Build value for perch position by asking for hand touch while maintaining position
Build value for perching with all legs on the block (working compression)
Shape position where front legs are on a separate block as rear legs (working extension)
Shape extension where rear legs are higher than front legs
Build value for extension on blocks through requiring hand touch
Continue to raise rear block to the point where a slight hop is required to get rear legs on block
Value Building- Front Leg Perch
Pimg already has massive value for the perch behavior, including hand touching while in perch
Value Building- Compression Perch
Pimg already has massive value for the perch behavior in compression.
Shaping- Extension Perch
I will start by placing a two large blocks very close to each other. I will ask Pimg for a perch and reward her for front paws on the front block and rear paws on the rear block. This should be simple shaping based on her value for the perch behavior. Foreseen problems may be that she will attempt to get all legs onto the front block. If this happens, I will use a much smaller front block with a much larger rear block. I will C/T for any independent movement of rear legs to rear block. I will reset by tossing a treat away from the blocks. I will not proceed to the next level until Pimg can perform an extension perch with equal size blocks that are not touching each other.
Value Building- Raising Rear Block
With value from the extended perch position in place, I will begin raising the rear block slowly up. I will C/T for proper rear foot placement. Foreseen issues here would be in her inability to step up with her back legs. Accounting for this would be simply to adjust the height in smaller increments.
Value Building- Maintaining Positional Criteria
Before adjusting the rear block up a notch, I will require proper hand touching while holding position. I will not raise the rear block until I have low (near the floor) hand touches, or nose targeting if that behavior is available.
Shaping- Rear Leg Hoping
Eventually the rear block will be high enough that Pimg will need to hope her rear legs up in order to get her feet up on the block. For this I will C/T for an behavior that represents attempts to hop up. At first, she will not likely be able to hop completely up. I will click for effort remembering to only click average or better attempts.