Supplements like these are designed to fill in any gaps in the trace elements that the plants require for growth.
As far as dosage, I would go towards the lean side.
Start with half teaspoon per gallon and see if you notice any improvements to the plants.
If you do not see any improvements then it could be one of two things:
1. The plants are missing some other trace elements.
2. (Most likely) The plants were already getting what they needed from what you were previously feeding them. Any additional products would just be a waste of money at that point.
If you do notice improvements to the plants, then you can either stick with that dosage or try to increase the dosage a little bit.
But do not overdo it. Remember, the plants were doing just fine without them.
As far as feeding, there have been big debates over the benefits of foliar feeding for orchids. But my thought is that if you are already foliar feeding with other stuff then you can just add this to the mix.