This style of ventilation works great for me when I don't have a fan in the tank. The temperatures also play a major role in dew buildup.
Rule of thumb, the greater the temperature separation the greater the potential for dew buildup. Ultimately, this will also depend on your humidity as well. With an inside temp of +10 degrees, (ten degrees greater than the outside temp), condensation will always be present at humidity of 70%. If you reduce your humidity to 60% and you reduce your inside temp to +5 degrees, you will have less condensation on the glass. This is all due to glasses poor heat insulation properties. Having thicker glass or an additional insulator on your glass will further reduce this condensation. Or, as I choose to do, you can have a slow and constant source of cooler and dryer air constantly flowing inside the tank.
To "FORCE" a steady flow of cooler/dry air into your tank, you have to rely on the natural thermal property of air current convection. This also keeps air inside from getting dank and stale. The angled Plexiglas is the important part, as well as the vent in the back. The angled Plexiglas only has to come as far down as the black rim, with a 1/4 inch to 1/8 inch gap on the lower/front of the angle. This keeps the cooler air sticking close to the front of the tank, further exaggerated by the temp of the glass itself.
See the picture for more info...