Great question, Jeff. What I have done and anybody can tweak this to their preference is to use a multi-year range. You need a wide enough spread in the ratios to make this worth while and a switch should not happen often as the bid/asked spread with a dealer will bite into your growing stack of metals. Take a 3-5 year range of the ratio. You might wish to throw out the high and low ratio possibly as those would represent absolute market extremes. When within 10% of the high and low of the range, could be switch time. Another way to look at it is when the ratio narrows or widens enough to make it worth your while to switch. Almost intuitive feel for when one metal is over/underpriced in relation to the other. For example, at the present time, silver, I believe, is undervalued. Your in silver now.