The root touch-up I did today turned out too dark. How can I avoid making it look more patchy while trying to fix it?

I did a root touch-up for a regular client this afternoon. After rinsing, I noticed the area near her bangs was half a shade darker than the mid-lengths. It wasn't a major issue, but applying a strong bleach directly would easily make the ends even drier. I first checked her previous formula and today's processing time, confirming that the darker color was caused by the temperature at the…