Level range of zones
 

From the Prima Guide (found it on Reddit though)

Hinterlands 12+ and Storm Coast L13-15+ are related to missions that take place there. Might be the case for the other ones with multiple level ranges.

Haven - L1-4
Hinterlands - L4-7, L8-11, L12+
Val Royeaux - L4-6
Fallow Mire - L8-10, L12-15
Storm Coast - L7-11, L13-15+
Forbidden Oasis - L8-12, L5-15, L18-20
Skyhold - L10-12
Crestwood - L10-12, L13+
The Western Approach - L10-13, L14+
The Exalted Plains - L10-13, L13-16
Emerald Graves - L14-16, L16-20
Emprise du Lion - L16-19, L19-22
The Hissing Wastes - L19-23

And it's a bit misleading… IIRC the only creature level 23 in Hissing Wastes is a dragon. ;)
So if you want the "fair" match, just be around the smaller number. No real need to pump up to level 23 for that area.
Unless of course you're playing on nightmare difficulty. :p

Storm Coast is also a lie. There is a dragon there level 19 IIRC, so they should have put x-19. ;)

Dragons have level scaling somewhat outside regular scaling, so I wouldn't really take those into consideration.

Essentially, I think it's as simple as this:

Low level: Hinterlands, Fallow Mire, Storm Coast and Forbidden Oasis.
High level: Emprise du Lion and Hissing Wastes.

Everything else scales enough to be considered "the middle", which means it can basically be done in any order.