Adjusting cooking time and temperature when making smaller portions
There is no good formula to calculate the time needed for baking custards. It depends on way too many variables, most of which you cannot know, and the calculations would be way too complicated too (a system of differential equations, IIRC). So the sensible way to go is to monitor when it is done and remove it exactly then, not to try to predict the time. 

The easiest way to monitor it is by using a thermometer. Without flour, you aim for 83 Celsius final temperature. With flour, it is something higher, maybe 90 Celsius, it will depend a bit on the proportions. Simply stick a roast thermometer into it and bake until it's there, then remove from the oven. 

As for the temperature of the oven setting, the lower you can go for a custard, the better. Depends on your patience. I've made creme caramel at 120 Celsius, took close to 3 hours, but the result was great. Setting it around 150 is more common, no matter the amount you are baking. But try not bake a single very deep dish, either make it flat and wide, or use multiple dishes.