You don't need a completely standard way of doing anything once you locked yourself into FTNxx by using Clearwin+ - you just need a way that works, and tells you explicitly just in case SF changes the rules.
The best way of doing this is not through a compiler setting or command line option, but with an OPTIONS directive in every file.
OPTIONS (DREAL)
If you try to compile this source code with a different compiler, it will baulk at the line.
You might even help someone in future by adding a comment:
OPTIONS (DREAL) ! This makes all REAL variables DOUBLE PRECISION in FTN95 (non-standard)
Who knows, you might even add comments to further describe the issue solved:
! The right answers are not obtained with low precision REALs
For mecej4, I suspect that the round-off error was cumulative, as the results shown in the graph slowly diverge. It's that, rather than the 'big error' at the end of the procedure that count.
Eddie