Negative Value with a slider

I create a slider to change a value : the VDF Cutoff of my Korg M1.
On the M1, the value range is -10 +10
I use in CTRLR a slider.
I put this SysEx formula : F0 42 30 19 41 00 09 xx 00 F7
The values are change fine to the positive value (0 to +10)

My problem : the negative values doens’t work
i don’t know how to manage the negative value (7F and not 00)

My values / SysEx :

10 F0 42 30 19 41 00 09 0A 00 F7
9 F0 42 30 19 41 00 09 09 00 F7
8 F0 42 30 19 41 00 09 08 00 F7
7 F0 42 30 19 41 00 09 07 00 F7
6 F0 42 30 19 41 00 09 06 00 F7
5 F0 42 30 19 41 00 09 05 00 F7
4 F0 42 30 19 41 00 09 04 00 F7
3 F0 42 30 19 41 00 09 03 00 F7
2 F0 42 30 19 41 00 09 02 00 F7
1 F0 42 30 19 41 00 09 01 00 F7
0 F0 42 30 19 41 00 09 00 00 F7
-1 F0 42 30 19 41 00 09 7F 7F F7
-2 F0 42 30 19 41 00 09 7E 7F F7
-3 F0 42 30 19 41 00 09 7D 7F F7
-4 F0 42 30 19 41 00 09 7C 7F F7
-5 F0 42 30 19 41 00 09 7B 7F F7
-6 F0 42 30 19 41 00 09 7A 7F F7
-7 F0 42 30 19 41 00 09 79 7F F7
-8 F0 42 30 19 41 00 09 78 7F F7
-9 F0 42 30 19 41 00 09 77 7F F7
-10 F0 42 30 19 41 00 09 76 7F F7

Sorry for my poor english (i’m french) and thanks a lot for your help !

Ctrlr