Use the range tool (push #2) and have your quantize set to 1 bar. Select the 8 bars in the volume automation lane and drag down. If you need an exact value, then switch to select tool (#1) and grab the two points you had just created and type in the value you want the new range to be in the info line.
If you want more points, they will snap to whatever quantization setting you have. I have key commands set for whole, quarter, eighth and 16th note quantize to just quickly switch to whatever quantization I want. If you’re not that familiar yet with range tool-check it out-it’s an easy way to create the edits you want and copy/past them and drag them, etc.