So we created GridGuide a while back to use originally as an internal tool but we didn't see any reason not to make it public. I've posted it here before but we've just launched a nice little update so I thought it'd be worth sharing again.
It's a small tool to help generate whole pixel grid values. Entering a width and a column count and it'll show you all combinations with whole pixel values with draggable PNGs.
A real world use case was earlier today for me where I needed to calculate values for 4 columns within 9 of the projects 12 column grid.
Aside from freshening up the design we've now added in a ratio selector for the outer gutter. So perfect if you're using Sketch. We use it internally a fair bit but we're always up for adding in new features so it fits your workflow.
Let us know what you think :)