That is the calculator I use. There are a host of other factors. But VHF is still close to line of sight. The higher dB signal will reach slightly further, but not much, it will be stronger at that limit. But heeling or listing, will decrease the strength of a high gain antenna, since the signal is focused in a horizontal plane. 6 dB is the usual recommended antenna.
The other factors have to do with SWR--if the SWR is elevated, the output of the radio will be reduced, and the antenna will not be as effective. Not all antennas are equal. Many of the Marine VHF antennas, are half wave, some are 5/8, some have base loading, and some have a stub. Some have better quality solder joints (they are basically just a wire inside of the fiberglass tube). I have found some antennas off the shelf with a high SWR, so I take my antenna tester to the store with me when I buy the antenna. My personal opinion is the "Digital" Brand is slightly better than the Shakesphere antennas. In either case buy the best of the lot for the best quality signal. If you want top end consider Comrod--that is used on a number of military and LEO boats.
The connections--including the PL 259 connector are another issue. I still prefer a well soldered connector. There are a number of cases of decreased signal because of the screw on/crimp on PL 259 connectors.
The coax from the radio to the antenna is also important. Most antennas come with a middle of the line coax and it is not worth changing on our small boats with short runs. You can cut the coax, as long as you leave 3 feet between the antenna and radio. Also it is advisable to keep antennas at least 3 feet apart.
Also the voltage to the radio. As I set up my new boat, I am putting a dedicated circuit directly from the battery switch (with fuse/breaker) to the radio. The radios are rated at 13.8 volts. If you have 11 volts (due to corrosion, poor connections, under sized wire, the signal output will be significantly less.
