Omitting the leading zero always gives an error...
Try "0.1+0.2" (no quotes).
If you want an incremental move, you can also do: "X1+0.2" and such.
FYI: the names of parameters don't usually match, between the three places you can access them: they are often named differently between Parameter Manager / PCB List Panel, PCB Inspector, and PCB Filter. AFAIK, there is no comprehensive list of Inspector names, so you will have to guess a lot.
You can also do more complicated assignments, on certain (usually text) fields, when a selection gives you a "..." button on that field. The syntax refers to the same (Inspector) parameter names, and functions are Pascal style.
Tim