Adjusting cooking time and temperature when making smaller portions I'm perfecting a recipe for queijadas de leite - a traditional Portuguese custard/pastry. It's a mixture of eggs/sugar/milk/flour, very liquid in its raw form. The original instructions call for 30 minutes baking at 350 ºF, and this works quite well when using full sized muffin pans.

I'm trying to adapt it to mini-muffin pans (which are about 1/3 the volume of the full sized), and find that without any modifications they get overdone. Any suggestions on how to adjust the temperature (and perhaps duration) without having to do 10 different batches?

Part of the difficulty is that milk is added at near boiling temperature. If I make lots of batter and try different settings for smaller batches, I'm afraid the cooling of the second, third, etc batches prior to baking will affect the results.
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.