Tube are high voltage, right? Do something to increase the clearance between the ground plane and the traces, either in the drc , or via "class", or "change isolate" on the polygon.
There are a couple ways to measure. Use "mark" and the status bar will show the distance from the mark as well as absolute coordinates. And there's a measurement tool. Or what I usually do is draw a line on some innocuous layer, and the use "info" to see how long it is.