You are having a hard time understanding the fundamentals. Your string is truncated because you are not requesting enough bytes. Each character is a single byte. Increase your byte count to something arbitrary such as 1000. You'll get the full string. No need to try to guess how many bytes are supposed to be returned. The read will stop with an EOI.
You may also need to use "%e" in place of "%f" in your format string. That will interpret the string as representing a number in scientific notation. Look at the detailed help for the Scan from String function and follow the link there to the format string help.
