I believe this is not supported by the format! macro (therefore no supported by println!), a workaround of this is to format as a String and then find the position of the e character, then insert spaces as you want
Well, it's a quick example, so it doesn't properly handle e.g. the lack of precision/width (note the .unwrap_or(0)s in the code). So ideally, you'd review and improve/extend this so that it handles edge/corner cases correctly as well.