I just saw the part where you are taking off tomorrow... I think your rule of thumb for EI will work fine.
You can finish film testing when you get back. The meter readings and interpretation that leads you to Zone System processing notations (e.g., N-1, N and N+1) will not change if you change developer or times in developer later.
The only time this strategy (test later) failed me was when I rated some expired TRI-X at EI 320 when it later developed and proved to only be capable of approximately EI 64. So I had insufficient camera exposure to overcome fog. You will not have that problem.