What does VARIABLE feature do on DL750?
In terms of using VARIABLE on your input channel;
it is a display-value feature - not a resolution feature.
the A/D on the front-end still uses the V/Div which you had previously set.
Here is a different approach:
Divide your signal’s Vpp by 20 divisions; and be sure to adjust your V/Div no-higher-than that value.
Now, your waveform will be off-screen a bit, by as much as 10 divisions-worth of waveform that you cannot see;
No problem, simply adjust the V Zoom to a value less than X1. This keeps the A/D in the best resolution mode; and, this will improve your SNR (ratio).
You may also use the VARIABLE feature after you setup according to 20 input divisions (veritcal).