The maximum certified altitude for the ATR 72 is 25, 000 feet. It’s altitude capability at any given time can be limited to something lower than that and is a function of weight, speed, and temperature. I haven’t checked which model of ATR 72 is being simulated, but in general, you should be able to reach 25,000 feet at weights up to 17,000 kg at ISA and 170 knots. At ISA+10, you would be limited to weights below 14,500 kg. At 190 knots, you wouldn’t be able to get to 25,000 feet.
Above 20tonnes or 20000kg its pointless to cruise beyond 18000feet. Typical cruise over here is within 14000 to 16000feet. Below 18tonnes, 17000-18000feet cruise is achievable without forcing anything beyond notch.
The only rare circumstances to get high cruising at above 20000feet or more was it being empty loaded no passenger or cargo and it was a 3.5 hour ferry flight to a destination for heavy maintenance schedule with the recorded gross weight below 16tonnes.
I flew older model ATRs in the US Northeast a long time ago. We typically stayed below 15000 feet. In the sim I have flown it at 17000, but it won’t make it above about 220 kts. I was flying sim over Colorado so that much altitude was necessary.
Having an issue with ATR regarding performance I’m at FL200 power management in CRZ position and throttles in Notch but i cant maintain airspeed i’m just dropping lower and lower going down to around 170 IAS so i’m having to keep Power management at the MCT position.
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I’m used to generate the flight plan in Simbrief and in most cases I’ll fly 2000ft below the SB recommended altitude and that’s enough, never had issues reaching or mantaining cruise level.
I know this is a little older but thought I would toss in some additional info to future researchers.
Remember that IAS (Indicate) is not TAS (True Airspeed)… 170 indicated is roughly 200 knots True at 10,000 feet.
And I cannot speak to Real Life but I can give data from the sim itself, so for simulation purposes this is fairly accurate. I am flying the 72-600 right now and gathering the data in MSFS2024-SU2 (live version).
I have a 72-600 at 48900 pounds (pretty heavy but under max landing weight), Notch, Cruise Power, Auto Condition – I switched on unlimited fuel so the weight isn’t changing on me. QNH 1013, Clear Skies (minimum wind knocking me around)
At 12,000 cruising at 220 knots indicated and 263 knots True (29.7 lbs fuel/min)
At 17,000 cruising at 201 knots indicated and 259 knots True (23.1 lbs fuel/min)
At 20,000 cruising at 190 knots indicated and 257 knots True (21.5 lbs fuel/min)
At 25,000 cruising at 167 knots indicated and 246 knots True (16.2 lbs fuel/min)
If you are willing to burn more fuel you could also go MCT and go a bit faster.
At 17,000 MCT gives 214 indicated, 275 True, 26.5 lbs/min fuel burn.
At 25,000 MCT gives 182 indicated, 266 True, 18.6 lb/min fuel burn << not too bad really
Based on this it’s easy to see that cruise fuel burn for the same distance is significantly lower (almost half) at higher altitudes. Of course, you burn more fuel getting up there… I suppose with a bit of effort you could throw together an optimizer but I would say take the distance and multiply by 30 (or so) and make that your cruise altitude in feet (maybe pick some minimum like 7000’)… 100nm = 3000’, 200nm = 6000’, 300 nm = 9000’, 700nm = 21,000 feet… applying common sense, terrain elevations, SID/STAR requirements, etc of course – but probably a good first swag for flight sim purposes. I can tell you that the 13,000’ simbrief picked for a 59nm trip is WAY too high 
Just to be clear power levers above NOTCH position, MCT and all those things are no go in real life, those are for non-normal use only. If speed cannot be maintained, then the only correct thing to do is descent.
We used to cruise in the FL160 - FL180 range mostly. FL250 only on very light flights or empty ferry flights. The ATR just won’t go that high when heavy.
In icing it is even worse, ATR recommends chosing a level with a cruise speed at least 40 kts above the icing bug (there is a QRH table for this), so you’ll end up even lower than the usual FL160 - FL180 range.
That it is certified to get up there doesn’t mean that it is able under all conditions or practical to do so. Also don’t expect 200 kts+ cruise at those levels, it might do 160-170 kts indicated at FL250.
Normal cruise altitude for me in the ATR 72 600 is anything between 17000 to 19000 ft, depending on length of flight. If it’s a very short island hop then usually cruise at around 12000ft. Once my climb rate starts to drop below 500fpm when in notch and power switch in climb, I know I’m getting close to maximum cruise altitude.
For info, this is an extract from the ATR 72-600 QRH (metric). In icing conditions stay below the bold line to maintain a margin of icing bug + 40 kts. In practice you can regard that bold line the practical ceiling, there isn’t much benefit climbing much higher, icing or not.
Although the table shows that FL250 is reachable, in some cases the cruise speed is below 170 kts. So when climbing with 170 kts you’ll never get there. That something is reachable according to the table does not mean that it is practical to do so.
According to ATR, the practical ceiling is reached when the average climb rate is 300 ft/min or 100 ft/min if single engine.
18T
20T
22T
refer to Simbrief Flight planner (route details list)