If you use the grid feature, you can scale it to the display size and get a grid at whatever spacing you want. So if your sample is 10 seconds long (info~ gives 10000 msec), and you want a spacing of 8, multiply the total length by the reciprocal (one-eighth) of that amount: grid 1250. Or you could cut it up any way you want with different values, though there’s a limit to how close waveform~ will draw the lines.
If you move/zoom around you might want to match the grid to the new length (use display start/length values to calculate), though the multiplier would stay the same. Depends if you want an absolute versus a relative grid display.