Measured according to 538:
https://projects.fivethirtyeight.com/polls/favorability/donald-trump/r/
The exact metric measured is "favorable" (and not "net favorable"). On February 8, 2024, Trump's "favorable" according to 538 is 78.5%. (His "unfavorable" is 19.5%, and his "net favorable" is +59.0%.)
The resolution decision will be made at any time the favorability goes below 30%, or as close as possible to the time of death (even if that excludes surveys from before the death that have not yet been added to 538).
If 538 no longer aggregates data on Trump's favorability among Republicans at the time of Trump's death, then a suitable replacement will be found (with consultation from the community). The core of the idea is to measure the general approval and loyalty of Republicans toward Trump over time.
I see that 538 has not provided updated favorability numbers for Republicans since March 2024. I'm scouting some alternate sources of favorability information, in case we need them, and I'm going to sell my positions in case a judgement call needs to be made. Here are my current two favorite alternate sources of data:
1) Gallup
79% favorability among Republicans as of December 2023
This matches pretty closely with the 538 numbers for that time period.
2) Pew
66% favorability among Republicans as of July 2023
This number includes "lean Republicans", which I think is making the number low. Prefer Gallup for now.