There's an interesting note near the bottom about the ÷ symbol. Even today in the Netherlands, a symbol like -/- (like a percent sign, but with minus signs on both sides of the slash) is used as a kind of emphasized minus sign, especially in money calculations in a vertical layout where otherwise addition would be assumed. I don't know if this is used elsewhere, but it seems probable to me that it derives from the ÷ symbol mentioned in the text.