If you are using the computer keyboard, there is no native way to input the notes at a fixed velocity rate (yet, have no idea what the future brings it has been requested quite a few times).
With scripting it is possible to capture keyboard input and affect the way data should be written into the pattern editor.
Without scripting, the fastest way is going to the advaced edit on the right side and divide the volume of all notes in a track or selected area by 2 after you have recorded your input. That would at least save you a lot of manual editing.