I am still a believer in "don't touch the bobbin tension" IMHO it is very tricky to turn the bobbin screw and a little turn goes can make a really big difference in the bobbin tension. and then it is hard to get it back right.I am not aware that there is a special mark on the bobbin tension screw that will help you get it back to factory preset.so how do you know where you started. To test you machine tension- sew a live o the DIAGINAl on fabric. gently pull the fabric at both ends on the line and see if/which side breaks, If the top thread breaks then the top tension is too tight, if the bottom breaks the bottom tension is too tight, and is neither breaks then tension is good. I will try adjusting the top tension only first before messing with the bottom tension svrew.