How to set up a slow cooling on an AB Veriti thermal cycler?
If I recall correctly (from when I interned at ABI), you can't directly set this in the Veriti interface.  However, (again, I'm not 100% certain -- I couldn't find anything to confirm this after 30 minutes of googling,) the ramp rate percent refers to the percent of maximum ramp rate for the heating block.

According to the product website, the max heating block ramp rate is 5 °C/s while the max sample ramp rate is 4.25 °C/s, so you need a ramp rate between 2% and 2.4%.