Is this correct?
yes, that is correct. the post_max_size should be slightly larger than the upload_max_filesize though, even though the upload_max_filesize is set to allow 20MB, the post_max_size is set to 8MB, this will restrict the size the form can send to the server to be less than 8MB.
upload_max_filesize sets the max size the server will accept from the form.
post_max_size sets the maximum size of the file + form data that the form can send.
any file over 8mb will fail to upload.
also the max_input_time or max_execution_time:
max_input_time: 60 seconds
max_execution_time: 60 seconds
will allow the form only 60 seconds to send the data to the server, if it takes any longer than that, the form will fail