Do I also take actual wind forecast into consideration ?
No - ETOPs are stated as "120 minutes" as an indication of the time flown on one engine, but the actual approval is for a circle of a specific distance.
Do I use the right formula ? 389 kt TAS + 10 % for GS in FL100 + or - winds in 100 ?
The operator selects a single engine cruise speed which will result in a large enough circle of operation around each adequate airfield. The higher the selected single-engine cruise speed, the larger the circle approved - but the penalty is the higher fuel load required to be carried to permit a single-engine diversion at the nominated speed from the most critical diversion point.
Or is a dumb formula 120 minutes at 389 kt GS ?
See above - the distance is different for each operator.
If the winds etc. plays a role could it be that a flight must be canceled because no alternate available in given 120 minutes ?
Wind doesn't play a role - for the reason you have stated - a scheduled airline would be constantly cancelling flights, based on the winds aloft, not to mention the pain calculating the correct area of allowed operation for each flight. As the "120 minutes" is a somewhat abitrary figure, based upon the statistical chance of a furthur failure in that time, the few minutes either way added by the wind is negligible in the risk calculation.
As stated above, though, the wind does need to be allowed for in the flight planning of the single engine diversion contingency fuel to ensure that the aircraft always has sufficient fuel to reach an alternate given an engine failure at any point along the route.
Does all the EU carriers have the possibility to add 15 % margin for ETOPS 120 so that it makes ETOPS 138 ?
ETOPs are approved on an operator basis. They cost money, so are not applied for unless you need the performance. 60 minutes is "standard". 120 minutes covers twin engine operations across the Atlantic. 138 minutes covers northern routes, when an airfield is closed due weather. 180 minutes give you Pacific operations etc. You apply for what you need.