Ya gotta get the tool.
I suppose one could make a tool if one had a tool to get the measurements from.
Let me explain what you are doing and maybe you can find a different way to do it.
With the rocker arm off, the the ball of the lifter extends all way. If one could measure the height of the top of the lifter from the surface of the head one would have a similar measurement.
Now put the rocker arm on and push the rocker to the bottom and release (actually we usually squeeze all the lifters in a vise before reassembly so that when the rocker is placed on the lifter it automatically depresses to the heigth that pushes the rocker against the cam. This is the measurement you are trying to establish. The tool sits on the head and the measuring pin drops through the hole in the rocker and measures the height from its base (the head) to the ball of the lifter. The tool has the increments on the side.
It is my impression that the lifter is supposed to be depressed maybe half of its possible travel. So if one can measure the height with no rocker and the height totally depressed (run the cam till the valve is down - if the lifter has been squeezed to remove the oil the lifter will compress when the valve is lifted). You now would have two extents of travel. Some where inbetween there is right.
Bosch Master, ASE Master, L1
33 years MB technician