Number can be entered in MillMage’s UI in many places and, for convenience, you can use up/down arrow key presses or you can click on the up/down arrow buttons on the screen. In many of these places, I’d find a slider easier to use.
I’m not sure what UI tech is used to implement MillMage.but I’m going to guess that it’s Qt. Perhaps a widget like this GitHub - BugelNiels/qt-value-slider: Custom widget for numbers similar to the number controls in Blender. · GitHub could be used with good effect.