Hello Ray,

I'm supposed to dilute the 1 teaspoon 19.5 - 27.5 - 12.5 into two gallons of water, instead of 1 gallon. That's the 50% or the "half-dose" that I was talking about earlier. So if the 1tsp/1 gallon has around 250 ppm of N/1 gal, then a 1tsp/2 gals will have 125 ppm of N/1 gal. I'm not a fertilizer expert but based on what I have read from the AOS Vanda Culture Sheet, (a) vandas are heavy feeders and (b) more fertilizer is needed in warm weather. Lowland temperature in the Philippines range between 23 - 34 degrees Celsius (73.4 - 93.2 F) so given the warm/humid weather we have, I guess my vandas can manage to have that much as I am not yet seeing root-burns (I water them first before spraying fertilizer solution)...

If I'll be doing the 20% as you suggested, then I'll do the following math:
if 1 tsp/1 gal ≈ 250 ppm of N/1 gal and
if we only need 20% of 250 ppm which is 50 ppm,
then 250 ppm / 50 ppm = 5.
By diluting 1 tsp or 250 ppm of N to 5 gallons of water, the resulting solution will give you 50 ppm/1 gal.
In the same manner, 1/5 tsp of the fertilizer mixed in 1 gal of water will give you 50 ppm/1 gal.

If we convert 1 tsp roughly as 5 grams, and 5 grams ≈ 250 ppm of N,
and we only need 20 % of 250 ppm of N or 5 grams of fertilizer,
then 250 ppm x 0.2 = 50 ppm; 5 grams x 0.2 = 1 gram.
50 ppm of N or 1 gram of fertilizer per gallon of water.
I think that's a pretty weak fertilizer solution for vandas...