How to make microwaved porridge thicker
There are 2 main points that will affect the thickness of your porridge : 


The oatmeal/milk (or water) ratio  
The temperature reached by the oatmeal. Litterature says the oat starch gelatinization (that gives said thickness) happens at around 80°C. From experience, higher is better (probably because heating isn't homogenous in a micro wave, and you'll need to be close to the boiling point on some parts of your bowl to reach at least 80°C everywhere).


So, what I would suggest is :  


Put your 40g of oatmeal and 220mL of milk, and measure the exact time it take it to boil in the microwave. If it's significantly higher than 2min30, and the result is thick as you want, just microwave it that long every time.  
If it's still not thick enough, add a bit more oatmeal untile you reach the desired thickness.