Water pressure calculation from overhead tank
There are many online calculators for this, such as:

http://m.convert-me.com/en/convert/pressure/mwater.html

A meter of water is about 1.5 PSI so 12 meters is only 18 PSI and would be pretty disappointing for water pressure. You will have to run around in the shower to get wet.

When no water is flowing, the size of the pipe and the horizontal run are immaterial to this law of physics. It is the height of the water column that creates static pressure. When water is flowing, the pipe size and length may reduce the pressure, but in this case (assuming 50mm is an inside diameter) you shouldn't see any reduction in pressure.

Good luck!