There are no guidelines for the "too low to shoot" if using at AT in the SLGS method, as it was developed before people started using the AT meter. You can use the human meter values, which are 200 as no shoot, if you are new. Over time, and as you gather data, that number can go lower, but never below 90.
The 68 that Chris refers to above, is the "take action" number on the AT, where we start feeding higher vpcarb to bring in their numbers. If you were following Tight Regulation, it would also be the reduction point. On SLGS, the reduction point is below 90.