Mmmmm. Let's see. 25% * 500 + 75% * 160 = 245 which is pretty close to your measured 260. I'd say your measurement is the correct one.

High chlorine levels should not affect the CH test. Chlorine levels above 10 ppm affect the pH test, usually making it read too high. High chlorine levels can also make the TA test go from blue to yellow instead of the normal green to red.

Richard