Sorry that's a crap answer now that I read it again.
A compressor map has 4 variables as I said:
* PR on Y axis (calculates/calculated from boost pressure: (PR/intake P) - intake P = boost)
* SC speed (thick lines in the middle, I'd probably agree with your equation)
* Thermal efficiency of the supercharger (as a %, heat loss is used to work out power consumed to make the thing go round, thin lines in the middle)
* Mass Airflow (calculates/calculated from power, torque or engine geometry e.g. RPM, # of cylinders, Volumetric Efficiency etc).
The maps are really suited to OEMs. The better your assumptions, the more accurate your answer. Some assumptions are easy (intake temp & pressure for example) but other essential pieces of info such as volumetric efficiency are only accurate from very thorough simulation or testing i.e. OEMs only. If you have the background knowledge then it might be fun to compare two superchargers in % terms, but I wouldn't use the amount of data that anyone besides Renault themselves have to try and predict power/speed/boost etc.
Working out something in the middle (speed or efficiency) e.g. "I want X power with Y boost, what gear ratio do I need"
* You calculate the values on the axis by measuring stuff on a running engine or calculations (e.g. the mass airflow equation in my previous post, or a MAF sensor reading on an engine dyno)
* You can then fairly simply draw intersecting lines to read off the stuff in the middle of the map for your corresponding X and Y
Finding something on the Y axis is much harder e.g. "I have a ratio & I want X power, what boost do I need" (and hence is that boost feasible with regards to intercooling blah blah)
* You basically guess the Y axis, read off the corresponding speed & efficiency and use it to calculate the X axis.
* You then calculate/measure X and see how close your Y guess was.
* Adjust your Y guess until the corresponding X matches your calculated/measured-by-another-means X (e.g. calculating mass air flow using engine geometry or by reading a sensor on a test engine)
Alternatively, you can find something on the Y axis by:
* Having a guess at Y & marking the corresponding X on the graph
* Note down the compressor speed/efficiency as this is essentially fixed for your desired RPM that you want your power to occur in, i.e. 4000 - 6500 RPM not 273743637437.
* You then have a different guess at Y but stay on the same speed/efficiency line and mark the new X. You now have an X that is too high and another that is (hopefully) too low.
* Then draw a line from both X to the chosen speed/efficiency and in the middle of them both (as one is too high, the other too low), draw a line to Y.
The "small" bit in the middle of all this is:
1) Maybe 10 simple-ish equations and quite a lot of engine data (you often need a starting pressure or temp, engine geometry etc which is far from simple) for all the "calculate" bits.
2) And the much more complicated task of compensating for the temperature change that happens when you compress the gas which is another load of, IMO, more difficult equations.
3) Actually deciding/remembering what 8 things you need to work out in order to draw the right intersecting lines can be IMO challenging & confusing (speak for myself haha!)