I'm lazy and do not want calculate total from harmonics. 
Hopefully the siglent will do it for us someday.

What your measure concerns:
Using flat top makes sense to me, as it got the best amplitude accuracy.
Also using averaging.
I see the values displayed in the screen upper right..
To be honest, in all the years I´ve used fft as it comes, adjusted as long as the output seems plausible to me, without thinking about it.
Something I want to change, so I´ve read the application notes from lecroy about setting up the fft.
Span = half of samplerate, OK got it.
"Delta"-f = 1/time per division *10 divisions, OK got it...
But there must be more when I look at your screenshot.
Your timebase is set to 5ms, so delta-f should be 1/50ms = 20Hz, but it isn´t, it is 23.84Hz....Why ?
Is it because of the amount of fft points, we can choose up to 2M ?
And what about delta-f in general, the lower the better ?