So, (assuming here) you are step mashing using direct heat. You are wanting to know what temperature to do your saccharification rest at for a Belgian Double? I'd say 154*F if it were me. Based on this assumption.
Most belgiains of decent gravity should be mashed fairly low for attenuation to be high and the addition of sugar would usually only be 5-10% for fermentability and to make the beer "digestable". Mash schedules vary greatly for commercially produced abbey and trappist beers. Some are step-infusion, some single infusion, decoction etc. Do what makes sense to you. Try to obtain "Brew Like A Monk" or the other books in the series. They're easy reading and very informative. It seems the sac rest(until conversion) is usually in the 144-50F range and usually has a mash out in the 155-65 range for about 20min. Some have a rest in the 115-120F range. If you feel adventurous you could dough in thick, infuse to get to sac rest, and decoct to get to mash out. Direct heat will work, but creates more like ramps instead of steps. To each his/her own. Hopefully your brew goes well.