Thanks, BEagle! That is kind of what I assumed, and ironically my sim development is always fron Llanbehr to Valley, as the TACAN beacon is quite handy there. Now I know how the Offset Computer should work, I can implement it properly between the TACAN data and the Nav display, when in TAC mode as you say.
I have read in the APs that when the Nav display is in DL mode you only have the TACAN range (via the counter), not bearing, and would use this for TACAN Air/Air mode, ie: to find a Victor tanker. There was mention that if the Nav display is in TAC mode, and the TACAN is in Air/Air mode, the range counter will work as expected, but the display roller blind will slowly propagate round in circles as there is no bearing information. I guess that switching to DL just stops this circular distraction.
However, that is post removal of the Data Link system, where before DL mode would have been used if the Data Link kit was installed... But Data Link research a topic for another PPRUNE thread.
Thanks again, Scott