I have a question on how to use the treadmill for uphill training. I come to trail running from the road and have lots of experience with running marathons in the flats, but very little with running uphill.

I am starting my second iteration with the 50K cat-1 20-weeks training plan from the Uphill Athlete’s book after successfully completing my first 50K ultra in January (Bandera 50K).

My target race in June is of the same length, 50K, but almost three times the vert gain (2600m D+ vs. 900m D+). I live in a moderately hilly part of the US, so I need to use the treadmill to hit the plan’s vert weekly targets.
Treadmill running has never been my favorite activity and I have never spent any time on it. I lack any serious experience and I am confused on how to use it, because the exertion levels I experienced on the few test runs I made over the last couple of weeks feel much lighter than comparable runs on the road or on the trail.

For instance: yesterday I tried a simple, short work out in (mostly) Z2 comprising:
0.5 mile WU,
0.5 mi at 10%
1 mile downhill at -2%,
0.5 mi at 10%,
0.5 mi CD at -2/0%

Total was 3 miles with about 525ft vert gain. Yet it felt easier than my regular neighborhood recovery run of 3.5mi/~420ft. Also, the 10%/half mile sections should have been more challenging than the much shorter and less steep climbs around my house I usually run on (they’re all about .25mi max at 8% max), yet they felt much easier.

So here is my question:

Should I take the incline values on the treadmill at face values when planning my training runs or should I consider “treadmill vert gain” as systematically easier than “real vertical gain” and perhaps add something to compensate?

Perhaps a treadmill 10% incline is only equivalent to a real world 8% incline (or 6%? or 4%?)