The power settings to use are clearly marked on the instrument, both for manifold pressure (the Ata gauge) and RPM.
Switch to the observer gunner/bombardier seat to get a good view of the right-hand side of the cockpit and you'll see three lines on the bezel of each dial. The green one is continuous power, yellow is climb power with a time limit (probably 30 or 60 minutes, can't exactly remember right now) and red is the "never exceed" value.
These visual guidelines correspond to the exact same limits that are described for the Stuka in the sim's manual. They both have Jumo engines after all, as well as the He-111H, so if you want to you can navigate to
(your hard drive)\steam\steamapps\common\IL-2 sturmovik cliffs of dover\manual, go to page 95 or thereabouts and use the values provided there.
I'm i'm not mistaken, continuous power is 1.1 Ata/2100RPM and climb power is 1.3Ata/2300RPM, but in any case you now know where to look for the correct info.
There's also a supecharger control in the Ju88, along with oil and water radiators. Hope this helps you starting out with it
