If your test compunds are proliferation agents then you should expect an increase in cell growth in treated samples compared to the untreated samples. However, if your test substance is a growth inhibitor/cytotoxic then you should expect a decrease in cell growth compared to the untreated one.

In either case use the values of untreated as baseline and then show a percent increase or decrease in cell numbers.

such as if you get an OD of 1.0 with the untreated samples and 0.5 with test samples then

The percent inhibition/toxicity = ((1.0 - 0.5)/1)x100 = 50%

good luck

@Sudeep Kumar In this case I've used PBS + MTT kit as negative control and PBS+Cells as positive. I used 1x PBS because to avoid the phenol red from the medium. Here my direct query is that I require a formula to drive the cytotoxicity value of compound treated and control. Thank you.