pH and temperature are the key parameters for controlling the enzymatic degradation of malt components.
If you don’t mash in a kettle, the temperature step to a desired target temperature usually is reached by adding hot water (step-infusion) or a boiling partial mash (decoction). Thereby you may have to take into account that -besides the mash- the mashtun has to be heated, too. This heat capacity can be considered as system dependent constant. A good starting point for estimating this value is the mass of the mashtun -without insulation and cold parts (eg. outer casing).
You calculate the necessary temperature or partial mash as follows:
Take into account the drop in starting temperature of the mash, stored in the mashtun (it depends on the insulation and therefore it’s a value of experience).
Note that the Excel brewplanner (available in the download area) can calibrate thermal mass, heat losses and heat input of your system. It thus can calculate temperature drops automatically for you.