Using the dial to control a cover position

[jruben4]

I'd like to be able to control the % position of the blinds (cover object) with the dial. Example - they are at 100%, I turn the dial two clicks, and they go to 98%.

My current attempt at the Service Data JSON:
{
"position":{{rotationPercent}}
}

The problem is the dial has some remembered state (like 0% or 100%) and if internally it's 0% and the covers start at 100%, a small turn of the dial (to say 2%) makes the blinds make a big jump from 100% to 2%.

Is there some JSON magic to basically say if the dial turns two clicks you subtract (or add) 2% to the current state to make it the new target state? like current_position - {{ticks}}?