Having done this job on other Diesel engines so I'll put in my two cents. When you re-tighten the nut after making an adjustment, the gap will change. So always measure with the nut tight. Start by measuring a gap before unscrewing the nut. Estimate how much to turn the screw (if adjustment is even necessary). Break the nut lose, make a TINY adjustment. Then re-snug the nut, and then measure again. Repeat as necessary. These things are very sensitive and the required adjustment is usually only a few degrees of screw rotation. You will probably need to turn the crank shaft at the front of the engine to position the cam. Always turn the crank in the direction of normal engine rotation (i.e., don't back up).
I probably cheated but I never removed the glow plugs or used the crank shaft TDC mark. I just turn the crank very slowly if the glow plugs are still in (or it may spring past the desired position). I usually turn the crank until one of the cylinder valves was open and then adjust the other one (with the cam lobe pointing 180 degrees away from the rocker arm tappet). Maybe the TDC method is easier for a tractor because if it's hard to see the valves and turn the crank at the same time. If you do the valves out of order, it helps to have pencil and paper (with a sketch of the valves, like in the manual) to keep track or which are intake and which are exhaust and which have already been done (by recording the before and after measurements). Otherwise it's easy to skip a valve or adjust an exhaust valve to the intake spec. or vice versa.
As a parting observation, I find the manual page confusing. First, you need to be sure you start at #1 cylinder TDC on the compression stroke (there is also a TDC on the exhaust stroke that would give the wrong adjustment). Second, It's not clear which valves to adjust at which crank position. I assume you adjust the ones with clear circles in the table at #1 compression TDC.