The 16" barrel will have the shortest time between nodes and therefore, at least in theory,a smaller window of near optimal exit timing.
Here is a table from Chris Long showing optimal barrel times:
http://the-long-family.com/OBT%20Table.pdf
Of course, knowing the best times doesn't help much without knowing what times you're actually getting. One more reason I love QuickLoad.
BTW, I don't think you want to subtract a cartridge length from the barrel. The vibration doesn't stop at the bullet start point or tip or cartridge mouth, it goes end to end.
Note that, if you're numbers are correct, multiples at/near ".5" would be near optimal, the shockwave should be back near the breach in that time frame. Your calculations seem somewhat close in one respect, when I did the math (last year, my uncle made a FileMaker program to spit out the numbers), most of my barrel times predicted by QuickLoad were in the vicinity of the 6th and 7th nodes.