Hello,
It is based on how much residual power you have developed, your corneal thickness, cylindrical power if you have any... These are calculated by computers , and then the prk is done.
Next Steps
If you have developed number again, wait for 6 months to one year, for the power to stabilize and for your cornea to heal. lubricate your eyes 6 to 8 times a day , lubricant drops are available over the counter.