Geoff,
Just discovered your site and read up on the spider ["See the Spider",
http://puttingzone.com/MyTips/spider.html]. Thanks for your plug and link to my site [The Palm Caddy,
http://www.golfputtgreenreader.com/].
Actually, much of what you explained was thought about upon the introduction of the PalmCaddy predecessor "The Greenreader". Unfortunately, no one could figure out how to use it, so I renamed it the PalmCaddy and vastly simplified it's usage. My feeling was that if I could get people to use it, then they could start discovering some of the things that were not explained like checking to see if the general slope was consistent with the slope between the ball and the cup. Also, there were originally adjustments to be made if one slope dominated another. For instance if the back to front axis was 5 degrees and the horizontal axis was 3 degrees. Once using the PalmCaddy these adjustments I thought would seem apparent to the golfer, but I chose not to include them for fear the product would not be used because it seemed too complicated.
Actually, the product is amazingly accurate when used as directed. Most greens are pretty consistent in the general slope, and very minor degrees of slope can be calculated as you approach the green. Many times it is very difficult to evaluate the slope when standing on the green, and the closer you are to the hole the more difficult it is to see the slope.
If a golfer takes the care to line up the putt from a good distance behind the ball and then double checks the slope from 90 degrees from that position a good read can result. But unless you know how to evaluate what you see by knowing the "spider" as you call it, then it's pretty much a waste of time.
Thanks again,
John Kanelous