In heraldry ordinaries are often based on the rule of thirds. So, for a saltire you would first diagonally divide the field into thirds, and then draw the arms 2/3 the width of the centre third. (Bet you didn't know there were guidelines for this sort of thing, did you?) I'm too tired to do the math, but I think that works out to about 20% of the height of the shield?? Anyhow, the big question is, what is the official colour of the Scottish flag?