I'm no expert, but as far I can can tell you'll have to do some math to always ensure the gridlines look how you want them to.
You can do this programatically using property nodes. You'll want the Yscale.increment property to set the scale, you may also want to read from the Y scale max and min properties so that you can do some math to determine the value you want for the increment.
Most importantly, you'll want to use the active Y scale property to switch between your two y scales. You'll want to be careful to avoid race conditions so that you don't set the wrong things to the wrong scale.