This is no different than normal CO2 carbonation, it just requires some basic math.
First, decided what level of CO2 you want in your beer. Use this chart: http://www.zahmnagel.com/LinkClick.aspx?fileticket=HuVGZ8tLaow%3D&tabid=81
I shoot for 2.5 volumes at my 39F serving temp (personal preference on both of those, no need to go any further. I like my beer a bit colder and like to let it warm up over the course of a pint, but I digress.)
At 39F and 2.5 volumes of CO2 I need 12psi for 100% CO2. With my beer gas mixture being 25% CO2, all you need to do is multiple by 4 (25%*4=100%). BANG. Done. Set my serving pressure to 48psi and let it sit and carb up. (you can shake it for a couple minutes as well when set at the pressure and it will carb up faster or you just wait a week).
That is it. It really is as simple as that.
Another example. You have a serving temp of 45F and you want to go on the high side with 3 volumes of CO2. Go to your chart, at 45F and 3 volumes you need a psi of 21 for 100% CO2. Multiple by you beer gas ratio (if it is 20% CO2 x5 if 25% x4). So for 20% beer gas, 21x5 = 105psi. Now that is a bit high, like I said.
Final example. With nitro, you probably want to go on the lower side (honestly I think 2.5 is pushing it but that is what I like.) so 2 volumes at 40F serving temp. Go to the chart, you find to reach 2vol at 40F you need 7psi with 100% CO2. 7x5 (for 20% beer gas) and you get 35psi.
If you have any more questions, post 'em. But it is so simple to dial it in exactly like you want with minimal effort there is no reason to roll or invert kegs. Just calculate your pressure, purge the 02, shake just a bit (optional), and let it sit for a couple days.