unfortunately the map guide image is down for some reason.
Basically each tier has a maximum level of mobs which spawn, so it's highly recommended to start lower than the max level as they take time to complete, as an example you can expect to generally level up 2-3 times in a tier 3 map, 5 times in a level 5 map and 8-10 times in a level 10 map etc.
The max levels of the mobs which spawn are as below:
tier 1 - 15
2 - 20
3 - 25
4 (lvl 5) - 30
5 (lvl 10) - 35
6 (lvl 15) - 40
7 (lvl 20) - 45
8 (lvl 25) - 50
9 (lvl 30) - 55
10 (lvl 35) - 60
11 (lvl 40) - 65
12 (lvl 45) - 70
13 (lvl 50) - 75
14 (lvl 55) - 80
In general maps level 40+ it's usually wise to use them very close to the minimum level you can open them, as they take long enough to explore that you'll need most of the levels
The stats considerred "good"
+Evil wells
+Monster difficulty
+Evil presences
+Treasure Chests
Exp is generally not cared about as it's faster to level up by whistling than it is to explore a +50% exp gained map in most cases.