Answer
$\approx 847\;\rm mmHg$
Work Step by Step
When you are lying down, the absolute pressure inside the brain and the heart is
$$P_1=P_{gauge}+P_A$$
where $P_1$ is the initial pressure in both the brain and the heart.
So,
$$ P_{\rm heart}=P_{1,\rm brain}=118+760=\bf 878\;\rm mmHg\tag 1$$
And when you are standing up, the pressure in the heart is constant and is then given by
$$P_{\rm heart}=P_{2\rm,brain }+h\rho g$$
where $h$ is the height of the brain and $\rho$ is the blood's density.
So,
$$P_{2\rm,brain }=P_{\rm heart}-h\rho g $$
Plugging the known, and do not forget to convert the last term to the unit of mmHg. where $\rm 760\;\rm mmHg=1.013\times 10^5\;P_a$
$$P_{2\rm,brain }=878-\rm \left[(0.4)(1060)(9.8)\;\rm P_a\;\left(\dfrac{760\;\rm mmHg}{1.013\times 10^5\;P_a}\right)\right]$$
$$P_{2\rm,brain }=\color{red}{\bf 846.8}\;\rm mmHg$$
