I think the way its explained in Schweser and CFAI is way to complicated. All you do is divide or multiply by the bid/ask number that results in the lower number. So using the example on pg 80 problem 15. Take 1M dollars convert to BP by dividing by the higher number 1.201. You are now converting to SF and its quotes SF/BP. So you have to multiply. Multiply by the lower number which is 2. Then to convert back to USD. Its quoted SF/USD. Again you want to get the lowest number so you divide by 1.651. All those drawing the lines b/t the bid and ask is such a time waster.