No you do not loosen the pump. The adjustment is the same way as normal with the adjustable pulley in the timing case.
You turn the engine until just shy of TDC. The dial gauge will not move for a bit. This is where the plunger is at its bottom. Zero the gauge. Continue turning until the engine is at TDC (by the notch on the ring gear). Note the dial gauge reading and that is the lift.
Pissing people off on the "net" since 1983.
Land Rover. Turning owners into mechanics since 1948.