For B/Mid/etc, it'll be a number between 0 and 127. For example, to trigger the center position, you'd put 63 (or 64), and for the far end position, you'd put 127. Think of any slider, or control that is controlled by a slider, in MixEmergency as a range of 0 to 1.0. Then, pick the position you want (e.g. 1/4 would be 0.25) and multiply it by 127 (the maximum value for the MIDI control value.