As much as possible is the correct answer, but as a rule, you should aim for at least 4.5mm of available curvature data from the center in all directions. If this is too difficult to achieve in the vertical meridian you should be fine with 3mm of data above and below the center, although this will depend on the orientation of the flat meridian, as it is the corneal curvature information along the flat meridian that is needed for OrthoK lens fitting. Fortunately, most astigmatic corneas are with-the-rule (horizontal flat meridian orientation), hence the flat meridian being your primary concern. If however, they turn out to have against-the-rule astigmatism (vertical flat meridian orientation), you are going to need to try and capture beyond 4mm above and below the center.
The actual coverage you need is dependent on the lens design you are using because flat meridian data is needed out to the peripheral landing zone of the lens. Some lens designs will specify the bearing chord or half chord, but if not, you can estimate the total bearing chord to be 2mm less than lens diameter, and the half chord is, well, just half of this. If this all sounds confusing, see the main figure for more detail, where a lens is superimposed along the flat meridian that in this example has an axis of approximately 20 degrees. The peripheral landing zone of an OK lens is typically around 1mm in from the lens periphery, hence using this as a rough measure for good data coverage you need for lens fitting. E.g. for an 11mm diameter lens you should be aiming to capture beyond 11mm – 2 x 1mm = 9mm / 2 = 4.5mm either side of the center.