Musta. If you want the most reliable contact with atc - monitor the nearest atc frequency as shown on your gps.
If you are 100 miles south of Charleville at 7500 you will not get through to the area frequency Charleville outlet as shown using the frequency boundaries but you will get through to the St George outlet which is nearer. I will say again. VHF is line of site. Closer normally the better.
The boundaries shown on the chart are primarily designed for ATC workload purposes.
If you want the most reliable way of getting a mayday call out. Use 121.5. Its monitored by lots of special transceivers flying at up to 50,000' to give maximum range . They are called jet aircraft.