Treadmill Pace Conversion Tables

2.5-3.0 is a lot of incline. I don't think treadmill to outside is that different, maybe 3-6 seconds per mile easier on a treadmill. I think kicking up to 0.3% or at most 0.5% negates it, but really I think 0.5% on treadmill is then a couple seconds slower than outside on flat ground. The only thing you're trying to negate on a treadmill is the fact that you're no longer running into air at X miles per hour. The faster you are running, the more wind resistance. Doing a workout on a treadmill at 5 minute mile pace and 0% incline is like running with a 12 mph tailwind outside. Kick the incline up just that fraction of a percent should negate it.

RUNPACES has a treadmill equivalency table in it. You can set it to either pace or speed and imperial or metric. The shareware version is free but you will need to reenter your data each time you use it. Go to the Tables and Tools pull down menu to find it.

http://www.runpaces.com/

Here is another website that might be useful

https://www.hillrunner.com/calculators/treadmill-pace-conversions/

I like this one by Brooks Hanson. Seems legit. I use it as a reference quite frequently: https://lukehumphreyrunning.com/hmmcalculator/treadmill_mph.php