How 18650 cells capacity is measured (what are the amperage and cut off voltage params)?
If there's a significant difference in measured capacity between 500mA and 1A discharge currents, you should stick to lower value, i.e. 500mA (or even less). Unless you're interested in capacity under specific load, you should aim to measure the maximum value, and cells typically have higher capacity at smaller discharge current.

The cut-off voltage cannot be reasonably estimated and should be taken from the datasheet (idem for the full-charge voltage). If the additional capacity between 3.0V and 2.8V is insignificant to you, you can stop at 3.0V for all cells (or even 3.2V, which already correspond to 95-98% discharge; IMO 5% precision is plenty for the purpose of sorting used cells). What you should avoid is discharging cells with 3.0V cut-off down to 2.8V.