Miller Orr defines the difference between the upper limit and lower limit as the "spread"

Spread =15-3=12

MIler Orr also defines the return point as the lower limit plus a third of the spread. In this case:

5+(1/3*12)=9

When the upper limit is reached, sufficient securities are sold to increase the cash balance back up to the return point.

then: 15-9=6 

When the lower limit is reached, sufficient securities are sold to increase the cash balance cack up to the return point.

then:9-3=6

Spread calculation: 3 {3/4*transaction cost* variance of cash flows/interest rate}^(1/3)