I have also flown the Litton 92 with the FMS interface and again the tolerance we used was 3+3T. IIRC, there was even a GPS interface with the FMC.
Yes, I think you and our Tristar pilot are talking about different things.
I think that 3 + 3T is always used as the criteria for removing an IRS for maint.(on the second occurance) We still use it today on modern laser systems. But nowadays it is really u/s if it gets to that limit, whereas in the 70s it quite often got near there.
Edit I didn't write @ I wrote a, but can't edit it!