Sometimes a pattern asks you to decrease (or increase) x amount of stitches evenly across a row. Now I don't know about you, but there are times when my brain just will not compute. I have however through the wonder of the Internet stumbled across just the page for these scenarios and here it is How to Reduce Stitches Evenly Across a Row. All the maths has been done for you and this handy calculator is just what the stitch fairy ordered in times of need. It is so simple to use and to me has been a godsend. There is also a link to the increase calculator on the page. Suffice to say I have this page pinned to my bookmark bar for quick reference and I would suggest you do it too. You just never know when you might be in need.
Stay safe and happy crafting