The main reason an engine is easier to tune when using nitro is because of the air/fuel ratios. Petrol has a workable ratio of about 12-15:1 which is quite a narrow range between rich and lean so you have to be quite precise. Methanol has a workable range of around 4.5-6:1 which is easier to set than petrol but still a little sensitive. Nitro though has a workable range 0f 0.5-2.5:1 which is very broad (meaning you almost can't go wrong ). When you blend nitro with methanol then the workable range falls somewhere in between the figures for raw methanol and raw nitro. So the more you mix the easier it is to get an acceptable tune.