I want to adjust the Thickness of the "Inset Faces"-Tool manually, since dragging is very inaccurate, and I want the exact same thickness on every face of the cube. I googled around but didn't find how to open some sort of menu to type in the value. Help would be very appreciated.
You can do like schustudrai says, or for more accuracy and options, make your inset and confirm it: parameters can then be edited on the bottom left in the "Operator" panel, or in a floating menu accessed using the F6 key.
You can also use snapping:
- Ctrl while dragging to snap to increments
- Ctrl+Shift for finer-grained snapping
- set snapping for all your edits using the magnet icon in the 3D view toolbar.