I found the LVAR to use on my Stream Deck for the decimals (Inner Knob)
Increase: INI_RMP_COM1_STBY_INNER_KNOB_TURNED_CLOCKWISE, Number
Decrease: INI_RMP_COM1_STBY_INNER_KNOB_TURNED_ANTICLOCKWISE, Number
Other aircraft are using the Key Event: COM_RADIO_FRACT_INC and COM_RADIO_FRACT_DEC