Try passing a culture object (i.e. InvariantCulture, if this is system-stored data and the format won't ever be different) to the overload that accepts one; your current culture may be set to something that expects a comma as the separator instead of a period (or similar).
You could also try
string x = (0.791794f).ToString()
just to see what it prints out.
Checking CultureInfo.CurrentCulture might be instructive as well.
(Also, sanity check -- I assume those quotes are from you, and not part of the string value themselves?)