@ bopperthijs,

Try the following: Take your greyscale bitmap output and put it through a math node. Subtract 127 from (or add  -127 to) it. Then plug that into your displacement node. This should give you your indent/outdent without moving the whole thing awayt from the base-plane...

PS: Note that the displacement node will take -ve as well as +ve values. -ve values displace "inwards". +ve value displace "outwards". 0 = no displacement.
By subtracting 127 from the greyscale, you are converting the 127 "flat" area to zero and the rest of the map accordingly :-)

Cheers,
Diolma