As of late February 2015 the calculator is in beta and will no doubt result in some inaccuracies. That being said, from my testing, the results are generally very accurate. If all input is correct and critical attack is not a factor, the battle results should be exactly that of the server. The tool will be updated as discoveries are made.
As you may know, DSF devs have made several major changes to the battle behavior recently. Such sudden and drastic changes would make the calc very inaccurate. If additional changes are made the calc will be updated once the changes are figured out.
Note that because of critical attack (see the Critical Damage section on the info page), for some battles it is not possible to predict the exact result that the server produces. This will most often not result in large differences, but if it happens to occur in a way that, for example, takes out a slot by killing just a few more ships, it could allow the next attack slot to move on to the next full opposing slot and could result in a large difference.
As mentioned in the info page, leaving out or misassigning techs and other values can result in large errors. From my experience this is the most common reason for large errors between a battle calculator and the server. So take these values seriously and do your best to assign them accurately.