Yes, that's how I did. I can create a rung 836, but the output is always 0.0000 while tool length is set as 7.250 (or any other value).
I tried as well "OEM 180 Machine Z coord - Z Work Offset - Tool length" with the same result.
In the end, I managed to get Tool Length by using OEM 42 (Tool Z Offset). It works like a charm!
And I got stock size by calculating values from other DROs.
I already used some other DROs that are not in pull/down list, and they work fine i.e 124, 126.