It may be a really basic question but sometimes things don’t follow straight math rules. Several sites recommend diluting 20-20-20 fertilizers by 1/2 or 1/4. I’m having trouble finding 20-20-20 but I can easily find 10-10-10 or 5-5-5. Are those equivalent to the 20-20-20 dilutons, respectively?
20-20-20 is twice as strong as 10-10-10 and 4 times stronger than 5-5-5 by volume. If 1/2 C of 20-20-20 is called for, apply 1 C of 10-10-10 or 4 C. of 5-5-5. Always follow recommendations on the package as to how much to apply. 20-20-20 risks burning plant roots which may be why you are having trouble finding it. If you have in-ground plants, you probably don't need a fertilizer with phosphorus as most soils have adequate amounts. Industrial farming is one place where phosphorus may be needed. A soil test through your county cooperative extension service provides pH, soil type and mineral concentrations already present. If there is deficiency or a pH not best for what you want to grow, it will recommended how much to add to compensate. The use of balanced fertilizer where the 3 numbers are roughly the same has fallen out of favor. Excess phosphorus is a major pollutant to watersheds and one cause of dangerous algal blooms.