That might work close to the equator, but as you go further north and closer to June 21, it won't work. The "idea" is that if the sun is 15 degrees above the horizon, then it will take one hour for it to set.
Additionally, the higher in altitude you are, the less importance is the geography below you, i.e. ocean or land.
If your question is to determine when "nightfall" is to occur, then perhaps a rough guess would be that a clenched fist at arm's length represents about 15 degrees of declination or one hour. For more precise estimates, one must know one's lat/long and the exact time GMT. A set of sunset tables would make the calculation exact.
Hope that helps,