Upload & Sell: On
+1 @ camera's are different ... especially between different mfr's. Some have more head room in them than others @ their histo's.
A reflective meter is calibrated to ASSUME that the luminace being metered is coming from an 18% gray reflectance and then calcuates the amount of light that should be falling on it to return such a value. If you measure a non-18% gray value with a reflective meter (handheld or in camera) and use that value for your exposure, it will render the metered area @ 18%. It doesn't really try to make things 18% gray, it just makes the calculation based on the assumption that you are pointing it at an 18% gray reflectance area. So yes, you either need to meter off of 18% gray ... or use your 'gray' matter to know that you didn't and adjust accordingly. The same applies for whether it is a handheld at 18%, or an in camera at 18% (or some other calibrated value).
An incident meter measures the amount of light falling on it. +1 @ using a sphere vs. a flat will vary how much light is being collected onto the meter. In that regard, it is reporting collected luminance. That is why I find an incident meter so valuable ... it can tell me the luminance level in the area I place it, irrespective of the subject's reflectance value.
Personally, I like knowing that the illumination in one area is say 13.6 EV, whereas another area is 8.2 EV. In that regard, I know that I have a 5.4 EV range in my luminance ... without trying to figure out if it is because I'm metering an 18% gray, black, white or specular area.
If my scene/camera can tolerate an additional 5.4 EV variance in illumination to expand the dynamic range that my subject(s) naturally have, then I'm good. If not, then I need to reduce it or open it even more (think gradient background). Obviously for something like copy work, you want that range to be closer to 0.0 EV variance. I'm not sure I know how to use just a histo to make such an evaluation. I guess you could use seamless, etc. then replace it with the subject.
Again, I realize that there's more than one way to skin this cat. I don't see a histo as being able to tell me my luminance levels, but rather a representation of the reflected values that have been captured ... which are subject to the reflectance values of the subject, i.e. specular highlights being tossed in the mix for evaluation. With an incident meter, I can choose to remove that variable from the equation.
This isn't always necessary, particularly if your goals are for working with less dynamic lighting. Imo, the more you are trying to get the most out of your dynamic range by virtue of your lighting arrangements, the more valuable an incident meter becomes.
The OP states he wants to become an AWESOME STUDIO photographer. To me, that means he'll want AWESOME command & control of his lighting. What that means to him will vary as much as we vary on our opinions of the subject. To him that might mean being AWESOME with his composition or AWESOME at expressing creativity or AWESOME at capturing a persons expression.
Knowing that the histo can be "hiding" as much as two stops to help "play it safe" makes it very plausible to not use a light meter. I just believe that a light meter can help you determine if you are lighting "into" those extra two stops ... or are going "beyond" those two stops ... or are playing it safe and "wasting" those extra stops (variable amount for diff cameras) in your lighting approach. Knowing that the histo is intentionally hiding things ... albeit for "safety" reasons ... is enough for me to want an incident light meter around (even if I don't use it for everything).
Must he have one to become an AWESOME STUDIO photographer ... nope.