I would use the diameters you have shown above as they are from the "Engine Repair Course Code: 168" which, at the start of the section on "Crankshaft Bearing and Cylinder Bore Dimensions" (page 46) mentions inspection of Nikasil finish meaning the data given in this document are for Nikasil engines. The cylinder bore diameters I provided could be for the later steel lined engines where slightly bigger piston-to-bore clearance could be required.

The diameters provided in the Repair Course are for the cylinder bores only, not for the pistons. Engine piston size is specified as "piston for cylinder bore xx.xx" and not by the actual piston diameter. The confusing part is the statement in the manual "piston / cylinder bore grades". The "piston / cylinder" refers to the grades of the two (which should match) and does not mean that the diameters provided apply to both. The diameters provided apply to the cylinder bores only.

The piston diameter is usually measured about 1 cm above the bottom of its skirt and in the thrust direction. Otherwise, piston diameter reduces slightly going up from the bottom of its skirt (because the piston heats-up more in its upper portion). Also, the piston skirt area is not circular but slightly oval. This is why there is no clearly defined "piston diameter" but they go by "piston for bore diameter".

If you measure your pistons as mentioned above, you can establish what clearance you have to the bore diameter. As a rough guide for aluminium blocks with Nikasil, the clearance should be somewhere around 0.06 mm (0.0025").