In theory that's correct.
In the real world, sometimes primaries will shift around in chromaticity as their luminance levels change and their also maybe issues where white doesn't exactly equal R+G+B.
When measuring color, the most basic way to quantify color is XYZ, a luminance meter only measures Y values.
White's X, Y and Z values for 100% should be the sum of the X, Y, and Z values for Red, Green and Blue.
CalMAN Lead Developer