Most people believe a 1% incline on the treadmill is equivalent to running off the treadmill. It depends on the TM however. Some aren't calibrated well or don't hold the incline.
By the same note: Running flat on a sandy or muddy trail would probably take a 1 or 2% incline adjustment on a hard dirt trail to reach equivalency of effort. Maybe another 1/2 to 1 degree incline when moving to asphault and another 1/2 degree moving onto concrete. Why? Because softer surfaces dissipate much of the force applied to them and when you push off a sandy or muddy surface your foot slips back some.
Think about it when you run on different surfaces. Briefly try to go from asphault onto smooth concrete. It's a faster surface~~you'll notice it immediately. Do you worry about equivalency when you switch from trails to roads? Then why worry when you go to a TM?
TM running is different. Your stride will be a bit different. You have to be careful, because running outside you will naturally slow as you tire. on a TM you have to be aware of what's happening and lower the speed. If you don't you'll overload your hamstrings.
About 20 years ago, an Alaskan runner won the women's U.S. Marathon trials after running all winter on a treadmill. Other accomplished northern latitude runners do most, if not all, of their winter training on treadmills and do fine come spring.
All you should care about is the quality of your workout. TM's are great because you can totally control your workout while avoiding heat, extreme cold, ice and snow, rain, mud etc. You can run a steady tempo pace for miles on a TM. You can run precise hill intervals.