As far as I can see there is no way currently to manually set the spacing of the grid lines. I think it would be great if that was possible.
The reason is that price often reacts to round numbers. So I could set my grid spacing to 100 and only get the round numbers. In addition, currently the grid spacing changes when I move my chart to the left or right and the chart rescales. So you never actually know what the grid spacing is after shifting the chart, until you look at the scale. That makes it harder to compare the size of moves based on those lines.
The reason is that price often reacts to round numbers. So I could set my grid spacing to 100 and only get the round numbers. In addition, currently the grid spacing changes when I move my chart to the left or right and the chart rescales. So you never actually know what the grid spacing is after shifting the chart, until you look at the scale. That makes it harder to compare the size of moves based on those lines.