Working with time and duration values

Looking for assistance.

Description of issue:

When deducting duration from Time, after an hour 00.00 and before 00.45, the value is not deducted correctly.

Walkthrough:

Working with columns:

"OpenHoursTo" as text value, eg. 

"DeliveryTime" as whole number eg. 15, 45, 0

Purpouse -  find the timestamp: OpenHoursTo - DeliveryTime

Step_1: Transform text value of column "OpenHoursTo" to Time, we get eg: 16:00

Step_2: Transform whole number of column "DeliveryTime" to Duration

So if DeliveryTime is 15, transformed value is - 0.00:15:00

Formula used: DeliveryTime = #duration(0,0,0, [DeliveryTime]*60) (NOT SURE IF THIS IS CORRECT)

Result:

As can be seen highkighted values, except the last one, give an incorrect result. Once the clock hits between 00.00 & 00.45, the values are not deducted correctly.

Any ideas how can I fix this issue?

Thanks, 
Ed