Great, it works! Thank you so much misohoza! 
And now I encountered the same problem as with the pitch-modulation issue I described in my other thread : in a Halion program containing a main layer with two sub-layers (A and B), I would like to have 3 knobs in my Macro Page, Knob 1 to control the Decay Offset for layer A, Knob 2 to control the Decay Offset for layer B, and Knob 3 to modify the Decay Offset of both layer A and layer B, but in a relative way, meaning that it will increase or decrease the Decay Offset of those two layers starting from the values set by the Knob 1 and Knob 2.
Is it possible somehow?