Outgoing and Incomming transformation for a channel?

I have an Epson projector for which I would like to control volume. I’ve patched the epson projector binding and added VOL command support so now my item looks something like this:

Number HomeCinemaVolume    "Home Cinema Volume"    (HomeCinema) {  epsonprojector="homecinema:Volume:ON,6000" }

Volume in the projector has values from 0 to 20 with 1 step increments. But in order to control it I must send i 12x or 13x increments, eg. :

Number to send -> actual epson volume
146 -> 12
134 -> 11
121 -> 10
109 -> 9
97 -> 8 

I can transform data from the device with profile and transform:SCALE or transform:JS but what about data sent to device? One way I though about this is using a rule:

Number HomeCinemaVolumeInternal    "Internal Volume"  {  epsonprojector="homecinema:Volume:ON,6000" }
Number HomeCinemaVolume "Home Cinema Volume" (HomeCinema)
rule "Map Home Cinema Volume From Device"
  Item HomeCinemaVolumeInternal received update
  HomeCinemaVolume.postUpdate(HomeCinemaVolumeInternal.state / 12.5 * 5)

rule "Map Home Cinema Volume to Device"
  Item HomeCinemaVolume received update
  HomeCinemaVolumeInternal.sendCommand(HomeCinemaVolume / 5 * 12.5)

But Can I do this without using the rule ? Ideally I would like the item have values from 0 to 100 and then somehow transform then

I don’t know this binding but based on the Item config it’s a 1.x version binding. The only way to do this without a rule is if there binding itself supports transformations. If it doesn’t the only way is Rules.