How to determine luminance %?

Question

According to http://www.workwithcolor.com/color-luminance-2233.htm, RED (#FF0000) has Luminance: 54%. and light pink (#FF8080) has Luminance: 89%. Our designers like it but

Answer 1

Check the formula here: Formula to determine brightness of RGB color

Luminance (standard, objective): (0.2126*R) + (0.7152*G) + (0.0722*B)

Put R=255, and G=B=0, you'll get 54

Edit: For relative luminance, divide by the maximum (255) you get 21%

For white, you get 100%