My possible exposure was having commercial sexual contact without a condom.
4th generation hiv test giving conclusive result after 28 days.so the results shown that you're not intected by hiv.
Your test is highly unlikely to change in the future. Retake the test at 3 months and expect a negative.
Why does my test‘s reference s/co is 0.5 while the others are 1?
That does not matter. All that matters is you tested negative.
Is a 3 months test necessary? Is recently test results not enough to say that I am not infect by HIV?
The test at 3 months is more optional for you than necessary. And since your anxious go for it. Expect it to be negative.