A "small pinch of slow release 13-13-13" probably doesn't add appreciably to your nutrient supply.
If it was a full teaspoon, which is about 5 grams, meaning it's 0.8g N. Spread that over the roughly 26 weeks of release and it's about 30 mg/week, but release from granules does not spread the nutrients throughout the container.
That's one of the quandaries of orchid growing - terrestrial plants put out a lot of fine roots and lots of root hairs, so get essentially all of the nutrition applied to the very absorptive medium. Orchid roots, on the other hand, are sparse and limited in extent, with the root hairs being microscopic, which greatly limited water and nutrient uptake from the coarse, non-communicating media we use.