Hi John,
As Gary stated the stock software does not have a tuning step size adjustment.? The KD8CEC firmware does, and it is free.? The CEC firmware works with the Nextion screen as well as the stock screen and a number of others.
Here is the link to the GitHub repository:
Be sure to select the correct file to use Xloader with the hex file, or to set the correct values in the ubitx.h file.
I would recommend XLoader if you are not familiar with the Arduino IDE:
73
Evan
AC9TU