I’ll start with the idea that we match the belly of the head to the rod, and the tip to the fishing conditions. But, it is very important to consider the carrying capacity of the belly of the line. To simplify, I don’t want to attach a tip that is overly heavy for the line to cast easily. As an example, many of the rods that I use in the summer and fall like belly weights in the 325-375 grain range, as a consequence I prefer to use MOW light or mediums, 10 foot density compensated tips 75-95 grains or 15 foot DC tips under 150 grains. It is also important to factor in payload. Just as we don’t want to overload the line with a too heavy of tip, taking into account the fly to be fished helps with tip selection. Don’t expect clean casting results when exceeding the ability of the tip to provide turnover. That double plume marabou that is almost impossible to cast? Take off the T-8 and try it on something with more mass. Also, bear in mind that this is a two-sided coin. I dislike using lighter weight tips with heavy lines, and with few exceptions, I generally avoid small lighter flies with heavier tips. The reason is too much force causing interesting casting developments due to extreme and rapid dispersal of energy.
Fortunately, RIO’s website has the appropriate belly weight corresponding MOW tip recommendations that help to determine a proper balanced selection (See page 9). By utilizing the grain charts it is also easier to make a correct decision when selecting either the 10 or 15 foot DC tips.