M will set to full power, m1/2 and m1/4 is half and quarter power, forget about the naturel light, the flash used as main light source will negate that, if you use full power the use the sekonic as flash meter, or use the auto mode, and get the group as tight as possible, check the distance and set the flash for the distance and use as if it was the only light source, you should be ok with that