Are we talking about bringing the gantry into plane with the bed or tramming the spindle so it is perpendicular to an already planar gantry? They are different things right?

Assuming you have got your gantry in plane already (helped by ensuring your long rails are planar to start with), I then use an iterative method to tram the spindle itself.

1. Level the spoil board with a smallish (10mm) endmill. It will be slightly scalloped if your spindle is out of tram but this doesn't matter.
2. Put a 90 degree bent bar in the spindle with a DTI on the end and sweep it across the bed, I used about a 200mm diameter sweep. This will give you the tramming error, correct the spindle by rotating it left or right / forwards backwards and then goto (1). Note that left/right has to be done at the spindle not by jacking up one of the gantry carriages or you will take the gantry out of plane. If you can't adjust the nod (forwards backwards) n the spindle then you can jack up the back or front of both gantry carriages equally.

That is how I do it anyway, I do something very similar to tram my milling machine.