Use your phone or laptop camera to measure and match the luminance of your bias light to your TV's reference pattern. No colorimeter required just point and draw.
Point your camera at the TV and the wall behind it. You'll draw two regions to compare luminance color is ignored.
Put a 5% APL gray field or 10% white window on your TV. Most calibration discs or test pattern generators include this.
Tap Start Camera, then drag a box over the gray test pattern on your TV screen, and a second box on the bias-lit wall behind it.
Dim or brighten your bias light until the luminance difference drops to 1%. The meter turns green and the audio tone becomes continuous.