Well, there's no perfect formula: one 25mbps is different from other 25mbps, some providers also set caps for file downloads and the distance from your computer to our nearest CDN serverfarm does make the difference as well.

Anyways, if you want to experiment with the default number of Downloader connections, I'd recommend to try downloading the file from My Account using web browser in the first place, to see what's your average download speed. In general you can assume that web browser download is an equivalent of one Downloader connection. Then simply multiply if needed.

For example on one of the very fast 150mbps connections here I can't get the browser (single connection) to download faster than 1.2-1.4MB/s. However, with 14-15 sessions on, I'm getting close to 14-15MB/s so the difference is significant.

I hope that helps. Just remember to pause and resume after each change to the number of parallel connections.