Precise cooking time: how to figure it out
It is very simple.


Note the time you start cooking it, and the flame/power setting/temperature. 
Check periodically to see if food is cooked
When food is done: record time and subtract from start time. 


If you cook the same dish a few times, you can average it out.  A lot of cook times are approximate anyway; you have to check the dish a bit before it should be done to make sure it hasn't cooked faster than expected.