Nothing will be completely failsafe because everyone's microwave, jar, oat grain size and even milk is different. It'll need a bit of trial and error. I prefer cooking in the pan because it allows much greater control and you get superior results. Weekdays, when I need to get to work I use the microwave. The most important thing (for most people) is to get the viscosity they desire.
The first thing is to prevent the volcanic eruption. The problem here is that the jar was probably too long and narrow and that milk was involved. I know from heating up anything creamy in the microwave - like custard - that milk froths up fairly quickly.
The best thing you can do to stop volcanic eruptions is to increase the width of the container and add the milk at a later point or after cooking. I add milk and frozen fruit like blueberries, bananas, raspberries etc. after cooking because it fixes the consistency of the porridge and simultaneously cools it down enough to start eating almost straightaway.
Whatever type of partially-cooked oats you use (rolled oats like 'porridge' and 'jumbo' are partially cooked to remain shelf-stable for longer, and 'pin-head' and 'steel-cut' are whole groats that have been broken and need soaking and proper cooking) need about x5 liquid to oats by weight. So 100g oats tends to need about 500ml water.
I add liquid to oats and cover my wide breakfast bowl with a Pyrex plate, then cook in microwave on high until it just starts to boil up. Then I put it on defrost for another few minutes until it rises and falls several times, but before it overflows.
I will give you my precise measurements, but as I mentioned above it will depend on the quantities you cook, the shape of your bowl, oat size and liquid:-
75g Jumbo oats
375ml water
dash of semi-skimmed milk at the end and stirred for about 30 seconds. I can't be bothered with stirring during cooking in the microwave. The defrosting helps with that because it creates natural 'churn' without overflowing very easily. Not as good as cooking on the stove, but it's a time saving compromise.