tach or hobb?

for phase 1 testing - do you go by tach time or hobbs time for the 40 hrs.

I'm getting close....

Dave
-9A FWF kit
N514R

I used flight hours--basically hobbs, less any ground run time for engine testing.

Technically Phase I is to be measured in "flight" time, so neither would give you this. Flight time is measured from the time the wheels leave the ground until they touch again on landing.
However, it's pretty difficult to complete phase I testing within the hours allotted anyway, so it really shouldn't matter.
In the end, It's your call.