If the shutter timer error is positive, how should the treatment time be set on the unit?

When the shutter timer error is positive, it indicates that the treatment machine is delivering radiation for a longer time than intended. To compensate for this excess time, you need to adjust the treatment duration accordingly. Therefore, adding the positive shutter timer error to the computed beam-on time ensures that the actual time the machine delivers radiation aligns with the intended dose prescribed by the medical team.

In practice, if you simply ignore the positive error (as suggested by one of the other options), the treatment may result in under-delivery of the intended dose. On the other hand, subtracting the error or multiplying it by two would further complicate the dose delivery and make it incorrect, potentially leading to adverse treatment outcomes. Hence, the most accurate approach is to add the positive timer error to the computed beam-on time to achieve the necessary dose delivery accurately.