Here goes with the long answer. Dimenthyl Sulfide is found naturally in beer and is a flavor component that is necessary. Too much DMS will cause the beer in final product to tast like cooked corn and will effect clearity. It attaches to H2O and as a result can be boiled off during the boil. To maximize the reduction of DMS you should have a rolling boil for 90 minutes. Again not all of the DMS needs to be lost and many all grain recipes boil 60 minutes. The books say that to assure loss of unwanted DMS you need to have a strong boil with an 8% evaporation rate durring your boil.
Your SG always be effected by the concentration of sugars in your wort. That is to say if you add or subtract more H2O you will change the SG. Obviously that can also be changed by adding more H2O, "top off" as most do in extract brewing to bring total wort volume up to needed final total. In all grain or any full boil the challange is to add enough H20 to start with that when the boil occurs and the H20 is lost due to evaporation you will have the exact total needed.
A rolling boil is not an exact. It is defined and can be achieved. The actual temprature is effected by altitude. My rolling boil occurs at a lower temprature than yours because I live at a high elevation and my boiling temprature as a result is lower. It is highly unlikely that if you boiled your wort for 60 minutes that you will have a problem. Pay attention to your boil and do what you can to get that nice constant rolling boil.
