I built a little tool to help calculate layout settings for Sketch frames – columns, gutters, margins, and offset – without needing to do the maths 
You can enter your frame width, number of columns, gutter size, and side margin, and it gives you the exact values to use in Sketch. There’s also a live SVG preview showing how your grid will look.
Try it here:
Sketch Frame Layout Generator
Would love any feedback. Hope it’s helpful!