I need to find a way to reference a subtotal relative to one of it's lines.

Example
Product # Salesperson Quanity
Percentage
1 Amy 10
**
1 Ron 20
**
1 Jim 25
**
Product # 1 Total 55
2 Amy 15
**
2 Ron 15
**
2 Jim 35
**
2 Bill 20
**
Product # 2 Total 85
3 Jim 35
**
3 Bill 20
**
Product # 3 Total 55

I of course want to calculate the percentage column by dividing the quantity
by the total. The problem is that the relative position of the total varies.
Is there a way to do this without using code?

Thanks,
Arun