I don't know how this 49% number came from, but if the injectors are clogged, it doesn't matter if its 49% or 65% or 24%... you just need to clean them!
Secondly, you don't need a scanning tool to determine if the injectors are clogged. You will also notice misfires, rough idle and bad fuel economy etc.. However, the OBDII scanner may return a DTC code (in case of Ford Focus, it will be P0201 etc.)
Even if one injector is clogged, you still have to clean all of them.
Finally, injector cleaning (aka tune up) is preventive maintenance. So it's something that should be done every 10000km or so, irrespective of whether they are clogged or not.