Wonder how you got on with this problem,How do you know the head is out of square,obviously you could put a clock either in/on your spindle and tram the table,but this wouldnt be exactly telling you the truth neither would the method your suggesting with a large square-as even if the head was square but the table was leaning one way or the other this would show an error on the clock or square-Often with milling machines people would not notice such a small error,maybe a slight trail when say a tipped cutter was passing over a face but most used millers do this,hence when you step over doing a large face with a smaller face mill, a small step is very hard to avoid-but it is usually looks a lot worste than it actually is.If needed this can be lapped out without much effort.A way of eliminating "lean" on the table is by locking the vertical column,once you get into posistion prior to final cut,lock the column this often gets rid of lean,and if your machine is an import it might have not one on as they dont make good machines.Adding a lock on the verical column might well eliminate such an error.I have seen much worste such errors eliminated by doing this.Would have though a small machine such as this would have the vertical dovetail machined wider to take a fixed dovetailed "gib" held in via tapped holes from side,that way you could shim the gib,nip it up-and then use the lock that should be on the vertical column or allow you to put one on and use it for nipping up on the final cut.
