Press the chuck end bearing cone and cup in place in the headstock.
Take the other bearing pair (cone and cup) and measure the height of the cup and record this measurement. Flip the assembly over and measure from the back of the cone to the back of the cup, that is from the small end of the cone to the back edge of the cup. Record this measurement. The cone could protrude past the cup, a - measurement, or sit inward a bit, a + measurement.
Have someone hold the mainshaft against the front of the headstock and measure the distance between the bottom edge of the cup recess in the headstock and the shoulder on the mainshaft. Record this number.
Lets say the measurement from bottom of the cup recess to the shoulder on the shaft is 2.000 inches
Now, take into account the measurement of the cone and cup assembly. It stick past the cup .060
Your measurement is 2.000
-.060
which give you 1.040 spacer length with zero end play.
If the cone does not protrude, you have 2.000
The cone is recessed .060
So you have 2.000
+.060
which gives you 2.060 spacer length with zero end play
Chill the shaft and warm the bearings. A little lube oil will allow them to slide in place. All you have to do is factor in your end play to arrive at a spacer length.