I think you are correct. The ARRL handbook uses the 468 (and that is the formula on the test) number to calculate the length of the dipole. The problem with that number is that too many people get anxious about cutting and cut exact length, then they end up being too short after they use wire length to wrap around insulators etc. From the insulator to the insulator, the 468 number is good; to calculate what to cut before wrapping the insulators the 492 is good to start out.
The MFJ-269 is about the most comprehensive antenna analyzer you can buy in the US. Bruce has the better Japanese version, made by Kuranishi Instruments
that is smaller and better rated. It is also $500 used.
I am currently looking at getting a used (READ:Thrifty) MFJ-259B so I can do exactly what we are talking about. These meters will tell you when connected to your antenna the resonant frequency the antenna will radiate most efficiently at. (Did I just end a sentence with a preposition?) Until then, my suggestion is to tune to the freq you believe the antenna to be tuned to, and check the SWR. No good? Move 10kZ up or down the band. Check the SWR again. Any better? Try the opposite direction, still 10kZ away from the starting freq. Better? Keep going until something starts to decrease/increase the SWR. Then fine tune.
However, if you get the MFJ-269 before I get a MFJ-259B, you may become my best friend.
BTW, I have a 20M dipole made, not sure what the freq is to start, but you can borrow it if you like, just have to solder coax to it. Or you could come over one day and hook your IC-7000 to my HF mobile antenna. I use a Hi-Q 5/80 RT. We can tune in 20M some weekend day and see how well that 7000 really does. That is my next radio after i sell my kidneys.
Info on the Hi-Q Antenna
The Hi-Q-5/80 ALL SS (All Stainless Steel)
Remotely Tunable/Motorized (RT)
6 thru 80 meters
The CAP and Base are Cad Cam designed and CNC machined out of Stainless Steel. The lower mast is 1.5" dia. SS tubing with SS mounting base with 3/8-24 thread. Cap has also 3/8-24 threaded hole for the whip. The Hi-Q-5 is designed to be used with the Hi-Q CapHat for MAXIMUM radiation Efficiency on 80 M. It is optimized for the 80 M band, however it performs great on the 60-40-20-15 and 10-meter bands. Two versions are produced, military, Hi-Q-5M and the Hi-Q-5/80 for the ham bands
It is extremely rugged with a LIFE time guaranteed. 12 or 24 VDC, remotely tunable.
Antenna height: 3 ft, no whip
Antenna weight: 7 lb (6061T6)