When calculating from Sketch to CSS, common wisdom to determine em is to divide by 1000. For example, -0.69 in Sketch would become -0.00069em.
This will not be consistently visually accurate. If your font-size is being set in rem, it is better and more consistent to use rem for letter-spacing as well.
In my case, I reset rem to 10 pixels (by setting body font-size to 62.5%), then divide sketch letter-spacing values by 10. For our example above, -0.69 would become -0.069rem.