It would depend on the phosphor used in the monitor. The IBM 5151 used the P39 phosphor which emits at 525 nm. Amber monitors often use P3 which emits at 602 nm.

What about the grey monitors? Are they just R=G=B from 0 through 255? Or do they have a certain phosphor wavelength as well?

Edit: It also currently uses the following ratios to convert the RGB channels to a single channel that is the brightness to use(0-255 as 3 inputs resulting in 0-255 in greyscale format to input to the white/green/amber lookup tables for brightness):
#define LUMINANCE_RFACTOR 0.2126
#define LUMINANCE_GFACTOR 0.7152
#define LUMINANCE_BFACTOR 0.0722

Would that be correct for those types of monitors?

White is multiple wavelengths. P4 seems to be common, here's a list https://en.wikipedia.org/wiki/Phosphor .

I guess "100% brightness" can only be an arbitrary definition, because it varies quite a bit between monitors. And just to illustrate the issue... if vertical deflection fails and all electrons are illuminating a single horizontal line across the middle of the screen, it will be many times brighter than any kind of "100%" you could get under normal conditions.

But even if you decide on some arbitrary "100%", there can't really be a 'correct' answer because RGB color spaces are very different from single-phosphor "color spaces", and I don't think there's a way to even compare the gamuts. The long and short of it is that you'd have to settle for an approximation which looks like it's somewhere in the ballpark perceptually.

Long ago I made some attempts here (some of them based on this), but their "correctness" is also subjective.