Why do folks here say that the shot glass method only works with OTO?

I'd think that by combining equal parts distilled water and pool water and proceeding with the DPD test (doubling the result) one would arrive at a rough estimate of chlorine. Taylor says this about dilution of the DPD chlorine test:

Quote Originally Posted by Taylor Technologies K-2005 DPD Chlorine test instructions

* If color is off-scale: Repeat test using 4.5 mL sample
diluted to 9 mL mark with tap water. Multiply reading by
2 to obtain approximate sanitizer level. If color is still
off-scale: Repeat test using 1.8 mL sample diluted to 9 mL
mark with tap water. Multiply reading by 5 to obtain
approximate sanitizer level.
Source: http://www.taylortechnologies.com/pr...er&Number=5136

Enlightenment appreciated.