Looking at the numbers, assuming Wood Memorial, Hammond Noll and Lighthouse East are planning on fielding teams at some point in the cycle, I anticipate right at 320 schools.
Also, Elkhart Central/Memorial will merge after 2019-20, so I would anticipate 33 schools in 5A and them to be placed in a 5-team sectional for one year.
You're close.
Minimum of 32 in 5A. Then work back 1-4A with a maximum of 64. If there are fewer than 320 schools, the smallest classes fill up first (e.g., 4A would have 63 schools, then 3A, then 2A, then 1A, and then cut again from 4A if there's still a remaining bye).
If there are more than 320 schools, 5A gets the overflow.