Upper bound for the change of argument

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Hi I am trying to calculate the change of argument. For example, if I want to calculate the change in argument of function F(s) along the horizontal line from a+it to b+it(suppose there is no zero of F(s) in this line) then we can consider $$\Im\int_{b}^{a}\frac{F'}{F}(\sigma+it)d\sigma$$ From here, lets say we don't know about the argument of function F, but we do know the upper bound of F,say $$\mid F(a+it)\mid\leq M , \mid F(b+it)\mid\leq M$$ . Then can I calculate like this? $$\Im\int_{b}^{a}\frac{F'}{F}(\sigma+it)d\sigma\leq\mid\int_{b}^{a}\frac{F'}{F}(\sigma+it)d\sigma\mid\leq\mid[\log F(s)]_{b+it}^{a+it}\mid\leq 2\log M$$