Ab initio number of iterations

I am running ab initio but don’t understand how the number of iterations are calculated. This post Ab-initio iterations asks a similar question but it wasn’t answered as far as I can see.
I have heterogeneity in my particle stack due to protein hinging and I want to run ab initio with 4 classes. Box size is 540pix due to high defocus of 3um so I have chosen a window inner of 0.4 and outer of 0.45. The final resolution to consider is 15A and initial 35A with initial lowpass fourier crop of 10A. How does cryosparc determine that 1652 iterations are required? Is there any way that I can speed up the calculations? What does the fourier radius step size of 0.04 mean?