If you already have knowledge of how a Large Steam/Gas Turbine works, a Large Plasma Turbine is fairly easy to understand. Just like the other Large Turbines, make sure that nothing is in the empty space 1 block in front of the front face of the turbine (where the "rotor" is visually showing), otherwise it can cause issues with the multiblock formation.
Optimal Flow (Plasma)
By now, you should know that each type of Large Turbine (Normal, High Pressure, Gas, Plasma) has a specific nominal EU output.
The nominal EU output of plasma is: Optimal Steam Flow * 40. This calculated value should be stated on the rotor you plan to use under "Optimal Plasma Flow." This is actually the nominal EU output that this rotor is capable of generating if given the correct amount of plasma at the correct rate.
Since this nominal EU output is in EU/t, we will then want to calculate just how much plasma we need to feed into the turbine (and at what rate) to obtain our nominal EU output out of our turbine.
Rate of Plasma
We want to know how much plasma we need to generate the optimal amount of EU/t from our rotor and turbine. The formula we care about to do this calculation is (Optimal Flow) = (Nominal Output) / (Fuel Value)
Fuel Value of Plasma
First, we need to know the fuel value of the plasma we are using. Let's say we are using Helium Plasma. In GT:NH, 1 cell of Helium Plasma gives 81920000 EU. We want the EU/L fuel value, so we divide by 1000, since 1 cell = 1000L. 81920000/1000 = 81920 EU/L. Now that we have a fuel value, we just need to divide this number from our Nominal Output to get the Optimal Plasma Flow.
Optimal Flow of Plasma
Let's say we are working with a rotor that states 192000 EU/t "Optimal Plasma Flow". The nominal output of this rotor is 192000 EU/t. Dividing by our fuel value of helium plasma gives 192000/81920 = 2.34375 L/t or 46.875 L/s optimal plasma flow.
This means that if our Large Plasma Turbine is fed 46.875 Liters of plasma per second, our hypothetical rotor will generate 3840000 EU/s or 192000 EU/t, before efficiency. Multiply with the Efficiency stated on the rotor to find the output. Ensure that the turbine's dynamo hatch is good enough to accept that much output, or the turbine will explode.
It is worth noting that since fluid regulators can only go up or down by a minimum amount of 20 L/s, our example would have to settle with feeding our plasma generator 60 L/s. This is within the 150% more-than-optimal plasma flow that a Large Turbine allows, so we would still generate 192000 EU/t, but we would be wasting some plasma. A better choice of rotor would alleviate this issue.
Fluid regulation with GT fluid regulators will not work well with Plasma Turbines. Typically, the optimal plasma flow for these turbines are in the 10-50 L/s range, meaning that there is a very high chance of wasting plasma (remember, GT fluid regulators only go up/down in increments of 20 L/s, or 1 L/t). For example, if you had an optimal flow of 31 L/s, your only choice with GT fluid regulators is to set the rate to 40 L/s.
What you can do instead, is use IC2 fluid regulators. These blocks require very little power and can be precisely tuned up/down, the lowest being increments of 1L/s.
Another useful tip is that the calculation of your nominal EU output from a rotor may be more than what is actually stated on the rotor. Say you have a Draconium rotor. This has a Optimal Plasma Flow of 29.296875 L/s. The game will round this up into 30 L/s, and recalculate the nominal output as such, meaning that the nominal output will actually be 122880 EU/t (before efficiency) instead of 120000 EU/t as it says on the rotor.