To calculate the sample interval that is required you need to take the time/div and multiply it by the number of divisions.
eg. 10ms/div with 10 divisions gives 100ms
The next stage is to determine the number of samples that you want to have across the whole of the time.
eg. 4000 samples
Divide the time by the number of samples
100ms / 4000 = 25microseconds
The next stage is to use ps3000_get_timebase, one of the parameters tells you how long a capture would take at the timebase you require for the number of samples you have stated. You will need to iterate in a loop starting at timebase 0 and incrementing the timebase value until the time is equal or greater than you need.