Raspberry Pi Octoprint power troubleshooting with USB inline voltmeter current meter lightning bolt

I always get a power warning lightning bolt when I use my USB buck powered off my 3D printers power supply. My printer has a 40 amp power supply so I doubt that is the problem. I was troubleshooting blind because I could not see in real time what the raspberry Pi was drawing. My USB buck and out 3.4 amps. More then enough for raspberry Pi 3 B+. I ordered another USB buck with a variable output power adjustment.

USB voltmeter in this video
https://www.amazon.com/gp/product/B075M3RKDV/ref=as_li_tl?ie=UTF8&tag=fintechcomm0f-20&camp=1789&creative=9325&linkCode=as2&creativeASIN=B075M3RKDV&linkId=33b3b1670510f318fce3730bb79ab61b