Be careful, it may not be as simple as you think. Firstly, it depends on if you are using page or block blobs. It also depends on what library you are using to upload the blob. For block blobs, the storage client has a default value of the maximum size of the block being uploaded (32MB) and will split the file into n blocks - each block will be a transaction (see Understanding Block Blobs and Page Blobs. You will also need to consider retries, and as you point out, listing, deleting etc.
I suggest you look closely at how you are backing up and find the size of the blocks - then do the calculations. Then do some controlled trails in an isolated account and see if you can reconcile the billing transactions against your estimate.