This is a very handy thing well known in Sketch and Adobe apps. You can just type in +(your-value) in any input field and the app does the math.
7 Likes
Additional and related request:
Increasing value by ±1px by pressing arrow keys.
3 Likes
Additional request to your additional request!
Increase value x10 times pressing arrow up/down and shift key, just like in many Adobe apps.
2 Likes