According to observation of the moon over the course of the month there are a range of phases, ranging between New Moon, First Quarter Moon, Third Quarter Moon, and Full Moon. The synodic period or lunation is exactly 29.5305882 days. The standard explanation of the phases under Round Earth Theory is given as so:
"It's probably easiest to understand the moon cycle in this order: new moon and full moon, first quarter and third quarter, and the phases in between. As shown in the above diagram, the new moon occurs when the moon is positioned between the earth and sun. The three objects are in approximate alignment (why "approximate" is explained below). The entire illuminated portion of the moon is on the back side of the moon, the half that we cannot see. At a full moon, the earth, moon, and sun are in approximate alignment, just as the new moon, but the moon is on the opposite side of the earth, so the entire sunlit part of the moon is facing us. The shadowed portion is entirely hidden from view. The first quarter and third quarter moons (both often called a "half moon"), happen when the moon is at a 90 degree angle with respect to the earth and sun. So we are seeing exactly half of the moon illuminated and half in shadow. Once you understand those four key moon phases, the phases between should be fairly easy to visualize, as the illuminated portion gradually transitions between them.
However, in order to see a full moon with 100% totality under a you would need to be looking at the moon's daylight side face-on. But according to the geometry of RET we would never see the daylight side face-on, otherwise the earth would get in the way of the sunlight. There should always be a portion of the moon that is unlit. 100% totality should be impossible, no matter how much mental gymnastics are done with the scale. If we are not looking at the daylight side face on, complete totality is impossible.
See the following illustration by İntikam from theflatearthsociety.org for further explanation: