I am learning the robe here so bear with me. When I set exposure manually can I rely on the brightness on dji app screen. I note that at the bottom of the setting screen, there is MM and it varies with + and - values when I am playing with shutter speed and aperture combination. Should I aim it to be zero to get the optimum amount of exposure?

Enable the Histogram - best visual guide to exposure.

Additionally to Frank666's tip you can also enable Zebra stripes to avoid over exposured areas

Hi DTK, I agree with Farnk, by looking at the histogram for a specific image a viewer will be able to judge the entire tonal distribution at a glance.  You can use them as an aid to show the distribution of tones captured, and whether image detail has been lost to blown-out highlights or blacked-out shadows.