You are facing two variables here.
Firstly developer
I have not used this particular developer. In general, it is best to standardise on either a developer that you use one-shot or a developer specifically designed for multiple use with/without replenishment. My standard developer is Barry Thornton's two-bath developer. He recommended using it for 15 films but I have found that, by replenishing the second bath, it gives consistent results for many films over a long period of time.
Mamiya 7 meter
For the past 11 years I have exclusively used a Mamiya 7 with 65mm lens. However, I use a hand-held meter because the built-in meter is very tricky to use. It is a quasi spot meter meaning you need to know where to point it for an accurate reading, hold the AE lock and then recompose. I have found that, for my way of working, the meter can be as much as three stops different from the reading I have calculated with the hand-held meter. This is generally because I have metered to achieve good shadow detail and, in the image I am composing, a very bright or very dark area falls in the centre where the quasi spot meter would take a (false in this case) reading.
The best course of action is to tackle the two variables one at a time.
Firstly, standardise on a developer that gives consistent results (one shot developers are particularly good for this and can be used economically).
Secondly, spend some time learning how the Mamiya 7 meter works and decide if it suits your way of working. If not then use the camera in manual mode with either a hand-held meter or use the inbuilt meter as a spot meter and interpret the results as appropriate.
Finally, it is well worth pinning down these variables as the Mamiya 7 is a fantastic camera to use.
Best,
David
www.dsallen.de