The thin savings is the percentage of space on the array that is thin provisioned.
In your case, you've got 54.09GB of volumes, out of a total of 648.77GB of volumes (it's actually 54.10GB and due to rounding, but I'll use your number for consistency)
That means that the thin provisioned space is (648.77-54.09=) 594.68GB
Thus the thin percentage is 594.68/648.77, which is 91.66% By the looks of it, the result returned is being rounded down, so you get 91%. (It should probably be 92%) Going backwards from that number like you've done is going to give an incorrect result due to the fact the percentage is being rounded - your math works if you use 91.66%, but is off if you use either 91% or 92%.
The thin-provisioning-ratio will give you the same number more accurately - 1-0.08338 = 0.91662, or 91.66%