I would rephrase it to:

Hey

<manager>

,

recently I have received orders to do <order 16:30> order16:45 which would have caused overtime. I thought about how I can smooth the assignment process in the future and here is a list of times that I need to finish orders:

• <How long it takes to take the forklift to the garage>

• <How long it takes to perform an order (described as accurately as possible)> If you feel like I am taking longer than I should please speak with

  <coworker>

  to verify that my timings are accurate. Also, keep in mind that sometimes unforeseen circumstances e.g.

  <insert circumstance>

  can delay my work.

Please let me know whether you need any additional information.

Best

<OP>

This leaves enough open that you did not give reason for conflict imho and if she goes ballistic, answer politely but CC another manager or her manager stating that you are unsure how to respond given that your times were accurate and that you are working towards a solution.