I think it's the same as the relationships between the octane numbers you see. So if the pump as 87, 89, and 91 grades, then the mid-grade will be a 50/50 mix (ratio 1:1) of the 87- and 91-octane fuel. If the grades are, say, 87, 89, and 93; then the ratio would be closer to 2:1 of regular:premium to get to that mid-grade.

Only reason I know this is because premium gas in California used to be 92 octane. So the three choices were 87, 89, and 92. When we dropped to 91 octane, a lot of the stations near me had to swap out the yellow sticker on their mid-grades to read "88.5". That was my clue that the ratio had not been adjusted yet.