Bill,
Not sure where you mean by 1/15.
The 1/5 is the total that the mineral owner will get, then it's divided by the acres each person owns.
Lets say, the well total production for the month is $3,000. The total acres that the mineral owners would get is 1/5 or 20% times 3,000. = 600. for the acres that are in the spacing. Now let's say the spacing is on 120 A, so you can divide 600 by 120 to see what each A gets = 5. Now if the 3 of you owned all 120, you would just divide the 600 by 3 and each would get 200. But, lets say, the total the 3 of you owned in the spacing was only 90 A. That would be 30 A for each X $5 per A. or $150.
I'm sure this is as clear as mud, maybe someone else can explain it better.