Halfway between 470 and 694MHz would be 582MHz (bottom of channel E35) = 51.5107cm, 1/2 = 25.76cm, 1/4 = 12.88cm
Channel 56 = 754MHz (750-758 in 8MHz DVB-T/2 Channel width) = 39.7603cm, 1/2 = 19.88cm, 1/4 = 9.94cm
Channel 42 = 642MHz (638-646) = 46.6966cm, 1/2 = 23.35cm, 1/4 = 11.67cm
Halfway between 470 and 758MHz = 614MHz (bottom of channel E39) = 48.8261cm, 1/2 = 24.41cm, 1/4 = 12.21cm
So a gap of around 23.35-25.76cm would be optimised for the middle of the new UHF TV band with the lower end being exact for the new RTBF frequency of E42. You could optimise at 19.88cm for just channel E56 until the switch to E42 in a few months maybe?