Here is how to tune script as per your needs:
1. xy.txt file must have x and y coordinates in decimal format with a single space between them, as I clarified earlier.
2. In checkpoints.txt file you don't need to save their private keys, why? that is whole point, because we keep starting 100 million or 1 billion pub keys' x coordinates which will work as 2 billions, so it is obvious that their private keys are from 2 to 1 billion or the last 1 billion.
3. There are 3 things that you can change, step size to be subtracted, number of steps, and number of iterations.
all these are in numbers not in points.
4. Finally, why script was blinking, is because it was loading checkpoints.txt file, In my case I had 8 GB RAM with around 5.5 GB checkpoints.txt file, on a dual core system. It was taking around half an hour before printing steps.... Be patient, if no error occur, it will start printing within half an hour.