Place the objective over a region containing mostly grey matter then hit "Grab" in the main ScanImage window. You should see images of the sample at each depth and hopefully it will look similarly bright at each depth. You can increase the ramp of the power with depth by making the depth constant smaller. Once you have a good value, it can be reused for similar samples. Likely no single value will be perfect throughout the sample. For example, in brains the signal is attenuated much faster with depth when imaging white matter compared to grey matter.