While I can't point you to a calculator on beer gas mixture I can point you to this Fact sheet from the Brewers Association.
The key to balance a tap system is figuring out the opposing pressures. The keg to tap resistance must be matched by the gas tank push pressure. The gas pressure puts in a little CO2, the liquid line squeezes a little out. However, in long draw setups where the draft line has a lot of resistance the push pressure would have be so high that a lot more CO2 would be dissolved into the beer. The amount of CO2 squeezed out would cause very foamy beer. How to solve this? Mix in some Nitrogen.
Basically, the Nitrogen will never blend into the beer. It is used as an inert gas to maintain the pressure in long draw lines. Any change in carbonation would come from the mix ratio. If the mix is too low in CO2, as the keg empties the beer will go flat since not enough CO2 is being forced in. If the mix is too high in CO2 then the beer would gain carbonation from the beer gas.
Your ratio of 30% CO2 / 70% N is more suited to a Nitro tap setup. Traditional nitro beers have an original carbonation level of around 1-1.5 volumes of CO2. The amount of CO2 needed to maintain this carbonation level is minimal PLUS the faucets that pour these beers have an extra plate that restricts the flow even more, forcing almost all gas out of the beer. Without that specialized faucet, the beer coming out would just be flat with maybe a few sad bubbles of CO2 but no thick head of foam that you'd expect.