Here's a picture borrowed from the FSM - page 16 of the PDF available on
www.startekinfo.com
The special tool get clamped around the bottom of the hydraulic unit.
A top piece gets screwed into this fixture so that there is no movement of the special tool fitted to the rocker arm.
A DTI is set with pre-load on the top of the special tool
The top piece gets screwed down so the hydraulic element is compressed and this is measurement "Z"
From this procedure I assume that the rocker arm under normal circumstances should be touching the cam lobe and the cup (55) that sits on top of the valve stem should be touching both the bottom of the hydraulic element and the valve stem.
My little cups are loose - it looks like I need to buy a set of feeler gauges (my others are in storage at the moment) and check to see how big this gap is. If necessary I'll have to remove a camshaft / follower saddle and measure the heights of the cups (55) and see if I have the 3mm or the 3.5mm versions fitted...
...to be continued