You’re welcome! I’ve not worked with Cal-Mag, just the Calcium Chloride called “Rot Stop”. Either way, a foliar application is much faster at getting results, as long as you’re careful avoiding high temperature application. According to this reference, you’ll want to adjust the Cal-Mag pH before application. With luck, it’ll already be in that range, but your water may affect its pH.
With root feeding, it will take a lot longer to travel up the stem, into the fruit as Calcium isn’t absorbed very quickly through the roots. I had BER on tomatoes before and the foliar application works wonders at stopping it. There’s no way to know without experimenting if it will help with the cracking.
My tomatoes aren’t ripe yet, so I don’t know if the cracking will appear. It hasn’t been CA hot, but days have been in the 90s with 70s at night. I water every morning. Before planting, I rototilled some dolomitic limestone into the row.