You can only use the EC readings to estimate ppm with mineral salts. Chitosan is an organic molecule which brings a good part of carbohydrate with it and that isn't detectable. As a fun&learning experience add common sugar to water and use the EC meter
The ppm are easier to determine with a scale. Weigh your spoon full of chitosan and divide it (in milligrams!) by 3.785 (if you're using US liquid gallons) and that's your ppm. A heaped teaspoon full of chitosan weighs probably a few grams, for ease's sake let's say 3.785 g. That's 1'000 ppm and that's used 1-3 times per crop cycle or year on agricultural crops. For everyday (or rather everyweek) use I'd try 50-200 ppm. The high doses are meant to really give the plant's immune system a kick in the a and have it full throttle in war mode (the treated plants likely have a pest or disease already and need a "cure"). That comes at a cost; you can't have it both, growth and defence (ask your president... well, HE might indeed say that economic growth and war can be had at the same time but that ain't true in biology LoL).
BTW the conversion of EC in mS/cm to ppm is a factor of 500 for US standards or in your case 50 ppm.