Than you for your suggestion. At first I could not get it working.
After some more tinkering I got the lever working with these settings:
for Variable: L:BKSQ_MixtureLeverPosition_1, Number
>> Yep, 'Number' must be included
Axis min: 0, Axis max: 1
>> 'with 100' the lever jumped from idle to max at once
Update value: Rounding 'Float'
>>if you leave it at integer it doesn't work
These settings work for the Piston Duke also.