'Renault Sandero - Fitting A Cam Belt' on youtube
11:14 at first the mark is here.
11:30 then, later, it has changed to being further down.
a) shouldn't this mark be at 6 o'clock on the crank gear?
b) did he make an error to begin with?
In general, how is he reasoning through this?
The simple answer to this is that you can make any marks you want on the pulleys before you take the belt off. As long as you put the belt back on with the pulleys in exactly the original position as you took the belt off, everything will be fine.
I am guessing that he manoeuvred the cam into a suitable position to mark it then added his own mark on the crank.
It is not a good idea to rotate the crank or cam with the belt off, since you need to ensure that the phase of the cam and crank are also correct, since the crank rotates twice for each rotation of the cam.
If this was a older diesel engine, you would also need to mark the position of the diesel pump pulley if it is driven by the timing belt, to ensure it went back in the same place.