There is no documentation on the actual calculation. The only information I can find was that the committee agreed to reduce the Isha angle to 16 degrees. I can some close with a little tuning (it's possible the differences also exist due to the calculation of co-ordinates - we have no idea what co-ordinates are used to calculate the the time for Stockholm).
With the settings in the attached screenshot (which you can also use in the API), we are mostly within 1 min of the times on the islamiskaforbundet website.
If you are able to get official calculation angles and any adjustments, we can make this into a method.
