Generally telescope power should be selected to provide at least 20/40 (6/12) (0.5) acuity through the device.

 
Why not prescribe a higher power for better visual acuity?
If we prescribe a higher power than needed to provide the desired acuity through the device, the field of view of the telescope will be narrower and there will be greater demand for the wearer to keep their head still to stabilize the image. So, while it’s not unreasonable to consider a higher telescope power than the minimum required, it may make adaptation to the telescope more challenging for your patient.

For a telescope acuity goal of ~20/40 (6/12) (0.5) suggested telescope powers are:
(These are suggested starting points for telescope power.)

Visual acuity through best correction (if needed)                 Suggested power(s)

20/50- 20/70                   6/15-6/20            .40-.32                                    1.7x

20/70- 20/100                 6/20-6/30            .25-.20                                   2.2x

20/100-20/150                6/30-6/45            .20-.13                                    3x

20/150-20/200               6/45-6/60            .13-.10                                     4x / 5x

20/200-20/300              6/60-6/90            .10-.06                                    5x / 6x