Topping will accelerate leaf maturation throughout the plant. Generally, the earlier you top, the larger and stronger the actively growing leaves will become. If you have interest in collecting the smaller, upper leaves, which tend to offer intense aromas (and nicotine), then top high--below the crowsfoot of the bud head. If you have plenty of time before first frost, then the choice is yours. My general impression is that regardless of whether you top at 18 leaves or much higher, you will end up with roughly the same poundage of cured leaf. Topping below 18 leaves may reduce the yield of cured leaf.
If you decide to top at a specific number of leaves, then count them from the bottom of the stalk, excluding the two round, seed leaves at the very bottom, if they are still there. Count them on one or two plants of each variety, then just make a good guess for the rest.
Of course, don't top plants from which you intend to collect seed, and be sure to bag them before the blossoms open.
Depending on the exposure to light during the nighttime, from neighbors' insecurity lights (or your own), a few varieties will grow many feet taller than they would under natural lighting conditions. Most varieties are relatively insensitive to this. (In previous years, I've had to use a 5' stepladder to reach the tops of certain varieties, due to bright lighting from a neighbor's yard.)
Bob
