Page Lookups Percentage - SQL Server performance monitoring
The metric is trying to measure the number of logical IO operations performed by each request, in average. If your server is, lets say, serving a busy web site, then is probably a good metric (perhaps 100 is even too high for such!).

The witness will only have internal activity. In other words it will touch data files internally, due to various background processes, despite never seeing any actual batch. As such all internal logical IO will be reported as caused by the few batches this server has seen. 

I would say that the metric Thomas is recommending has some value but only in a a workload similar to what Thomas had in mind. A server that has only background activity, like your witness, is not going to report anything meaningful with this ratio. A server that runs intensive DW reports may see only 1 batch request per hour and run millions of page lookups for that one request. An absolute number like 'must be below 100' is only valuable accompanied by smallprint disclaimers like must be bellow 100 if your workload has frequent enough batch request to establish significant statistical trends and each batch requests is expected to perform under 100 page lookups in average. You get the idea...