Some Mamiya Cs also have those bellows factor markings that are indicated by the parallax correction bar. Even the early ones that don't technically have them marked (like my C33) will give you an idea, as they still have the parallax correction bar. Yes; a very simple and good feature. I love my C33 for close-up pix (although I probably would not if I did not have the Paramender).
For the OP, these are the basic considerations that create the "problem":
1. Your light readings give you an exposure that is accurate at infinity, to which your lens' f stops are calibrated.
2. Amount of light falling diminishes (exponentially) as distance from a light source increases.
3. When a lens focuses on something closer than infinity, the distance between the lens and the film is greater than when it is focused at infinity. (Turn the focusing collar on any external focus fixed-length lens to see this.)
4. From your film's point of view, the lens is considered the light source, not the light source itself.
Now, apply this stuff to an actual situation:
A. Due to 3 above, when you focus on anything closer than infinity, you are moving you lens farther from the film than it is at infinity.
B. Due to number 4 and number 2 above, this makes less light fall on the film versus that which falls when the lens is at infinity.
C. Due to number 1 above, your calculated exposure will actually provide less exposure any time you are focused closer than infinity.
If you add more specifically to number 2 above, you learn how to calculate how just much light falls off using the inverse square law. This law means that if distance from the source doubles, the amount of light falling is quartered. Vise versa: If distance from the source halves, amount of light falling quadruples.
You need to state this as an equation so you can plug your measurements into it. You compare the square of the extension at infinity, where f stops are accurate, to the square of the extension at wherever your shot is focused. To state this mathematically, to get correct results with your calculations, the measurements all have to be squared.
"Compare" means state as a ratio, which can also be stated as an act of division. When you divide, you end up with a correction factor that tells you how many more times the amount of light that you needed at infinity you will need to make up. It will always be greater than 1x, so, that helps you remember that you always need to be dividing the longer measurement squared (actual extension) by the shorter measurement squared (extension at infinity AKA the focal length of the lens).
Algebra states that when you have something raised to a certain power divided by something raised to the same power, you get the same solution if you remove the exponents and perform the act of division, and then raise the result to the power of whatever that removed exponent was. This just means that you can remove a step by squaring the factor at the end rather than by squaring each measurement individually before dividing.
The minimum amount of compensation you can provide is, of course, limited by the precision of your exposure controls. If you end up with a factor of 1.10 in the end, you need 110% of the amount of light that you decided would give the correct exposure. That is not even 1/3 stop more. I guess you could estimate that 10% more with "in between" an f stop adjustment or by adding a little time if you have exposure times that are long enough to time, but for the most part, 1/3 stop, and often 1/2 stop is as precise as you can easily get. So, basically, you just decide what your cutoff is for when you add that first 33% more light (or 50% more light if you only have 1/2 stop precision). I simply round to the next highest or lowest 1/3 stop. This means if my factor hits the 1.17 mark, I round up to 1.33 and add the 1/3 stop, but below that, I go with the exposure that would be correct for infinity.
To get your measurements, it first helps to determine what your measuring spot on the lens will be. Go somewhere outside and focus on something that is at infinity focus. Something way in the distance, miles away would be good. Now, say you have a 150mm lens. Measure 150mm from the film plane and remember (or mark) where exactly 150mm from the film plane is on the lens case. In the future, that is the spot at which you take all your measurements. You know what the point of reference is at infinity, and to be accurate, you need to always measure to that point.
This is picking nits. You could just measure to the diaphragm and you would be fine.
So, you know the bottom of the division problem without even having to measure it. It is the focal length of your lens. You just need to measure the actual extension, divide it by the focal length, and square the result to get your factor.
For instance, say you are using a 360mm lens to shoot a portrait. You are focused on the subject. Measure from the film plane to your lens. Say it is 550mm. So, you divide 550mm by 360mm and get 1.53. Multiply that by itself and you get 2.34. You need to give the film 234% of the exposure you have decided is appropriate. Multiply the shutter speed by 2.34, or open up to let in 2.35 times more light. I always just call this 1 1/3 stops and am happy, but it is not technically accurate, I believe. Someone else should chime in with exactly why it isn't.