Unfortunately, PlexTools doesn't show absolute jitter values - the jitter graph has a floating Y-scale and the graph is positioned vertically without a fixed scale.
So you cannot directly compare jitter on two separate discs.
One trick that has been used here, is to burn a track on one burner without closing the disc and then burning another track on the same disc on another burner. Then you can perform a jitter scan to see which burn produced the lower jitter value.
Alternatively, something like PxScan/PxView can be used for scanning. You still won't get jitter values in terms of a percentage, or even values that can be compared between scanning drives, but you will get jitter scans that can be compared between discs.