can you try half one way and half the other and see what difference it makes?

I didn't realize the stuff was that simple. Will have to try it, too!

Rennet, like any enzyme will have a minimum/maximum temperature range outside which it will not work. Additionally it will have an ideal temperature at which it works best. Enzymes always have a proper timing, concentration and temperature for maximal efficacy. These temperatures are established by the manufacturers...so it is not the recipe, but rather the rennet info. that should be heeded. Rennet can be natural (from calves stomach), microbally produced (vegetarian), liquid, powdered, freeze-dried, etc. If you have no literature on your rennet, I suggest googeling general temperature for optimal rennet activity. Chances are cold milk will dramatically increase the required time for the rennet to work. Overly hot milk may denature the rennet, effectively destroying it before it can do its thing. Try a commercial rennet website. The Junket brand website says for example; "Warm milk over gentle heat 88ºF (31ºC), take care not to scorch." for American mozzarella. You may have to settle for general information like this, because halloumi is rare and unusual.