I would log wrist watch time. Tach is only used for engine mx times and is useless for logging time unless the engine always runs at the specified RPM. A timer merely helps you log it accurately. How do people log actual IMC time? Surely not with the Hobbs.
I've heard of "rules of thumb" like multiply by a factor or something for a conversion but as you noted it depends on power setting...
Also, are the times you noted for "wristwatch" in H:MM or straight minutes converted to hours/decimals.... That could affect your interpretation. Reason i ask is:
Tach: 2.92 Time: 2.66
Hard to get 66 minutes...
just my .02