I feel that I must let you know that dyslexic I am . So math has always been a problem with me and some times with origami.
This is done with a half square, (1:2). You fold one of the short sides in ( this is an arbitrary fold, depending on the size it can be about a ¼ inch, it does not matter very much). Then you divide the remaining rectangle into 6ths. Be sure follow the valley and mountain folds.

Now you take one corner of the short side of raw paper and fold it to the closest valley fold with the pivot being the closest mountain fold.

Using this new triangle as a guide, fold the other corner of the short side of raw paper over the newly made triangle.
This makes two fold lines that cross or bisect each other. This is a land mark that will help show where the long narrow side folds are to be.

Fold the long edge of raw paper that is closest to the new land mark to the land mark and unfold.

Now fold the model in half and unfold. Match the long side of raw paper with the other side that you just folded so that it looks symmetrical.

From this point, you follow your instructions on how to fold the Star of David. I have found that this is a simple way for me to fold the Star with any paper, anywhere, without the use of a ruler.