I have two items on the MDT display now. The percentage and a “plain” number (it’s Watts, so a positive integer that can go into the thousands).
The number works like this:
//Item:
Number WR_house_consumption {channel="mqtt:...", channel="knx:..."}
//KNX Thing:
Type number-control : WR_house_consumption [ ga="7.012:5/3/3" ]
The DPT 7 is okay as a display value on the MDT device 