It says to mix at 25:1 because back then the quality of 2-stroke oil wasnt as good as it is today. I think 50:1 would be a little extreme for just trail riding ,which i assume your doing, on that particular bike. Anything from 32-40:1 would probably be a safe bet. With big changes in mixing ratio you need to make the proper adjustments on your carb.
You will get a thousand different opinions on this subject. My opinion is that in that air-cooled bike you should run at least 32:1, maybe even 26:1. I had a '79 PE 250, very similar to your IT. I ran 20:1 with excellent results. 50:1 is simply not enough oil for the large clearances that those old engines run.
Now watch the cascade of opinions and arguments.;)
I would only argue with the man who told me to run 50:1.
I was running 20:1 in my 125 with good results. I ended up going to 26:1 but I won't go into why now.
125's need a bit more oil than the larger displacements. And like 125 said, you will need to adjust your jetting accordingly with any change.