Here's a sample Scotty Cameron Putter fitting.
Personally, I wasn't impressed with this fitting.
While collecting Cameron putters is a nice hobby if you can afford it, it certainly doesn't make you a better putter. Nowhere in the fitting did they check to see if the putter Spencer was using 'fit his eye' and he could aim it straight at address, naturally. IMO, that's a huge aspect of a putter fitting and buying a putter, period. And I also believe that aiming woes can cause those putter issues.
According to David Orr (www.orrgolf.com) his studies show that 54% of the golfers aim left of the target from 6 feet away. My guess is that Spencer does as well and that's a big reason why he cuts across the ball, to get the ball to the target even though he's aimed left of the target at address.
If I owned a pro shop that sold putters or managed a golf store, I would buy a laser gadget and offer free aim fitting for golfers so they could find the putter that they aim best.