I didn't do a test section. However, I also didn't take the flat iron up to 450 to avoid additional damage to my chemically treated hair.
It sounds gross, but maybe you could harvest hair from your brush for a bit until you have enough to make a hair swatch (people who henna start out with this to test the color). BKT the hair swatch and see how it comes out.
I've never done a test section either. But, I also had my hair done in a salon three times before I did the BKT at home. I did use a different formula than the salon though. I have never been one to do test sections even when I was relaxing. I probably should of done a test section on some of my hair colors!
3B dark brown shoulder length
Co-wash, Living Proof No frizz, Biolage Gel, Giovanni vitapro fusion leave in
Hey I just thought of something. The underneath side of my hair, the part closest to my neck is the most coarse so I thought I test out a small piece of that tonight. If I do that and it's fine then when I redo my whole head on Saturday will it hurt to do that same section again or do I need to keep it separate somehow?