From your comment I assume you mean AC voltage reading across pins 12-15? Doesn't make much sense unless all secondary voltages are given for a 220v input at pins 0-1. All taps would be in series with pin 11 being ground so:
11-12: 6v
11-13: 24v
11-14: 36v
11-15: 110v
Since you are supplying 110v, i.e. half of that, you would expect to get:
11-12: 3v
11-13: 12v
11-14: 18v
11-15: 55v
However since you are measuring the AC voltage at pins 12-15, you have to subtract the 3v across 11-12, in which case you arrive at approximately 52v across 12-15 which is pretty close. The actual readings you get will also depend on the accuracy or tolerance of the transformer windings, the efficiency of the transformer as well as mains voltage fluctuation.
In any case, bear in mind that if you are getting a 50v AC output, then by the time you have rectified this to DC you will get nearer 70v, which may cause damage to your CNC .
You might have been able to get closer to the desired DC voltage by taking the output from 14-15, in which case you would get 55v-18v = 37v. After rectification 37v * 1.4 = 51.8vDC. As I mentioned though, I don't believe the transformer will handle 300w.