Here is how I found the the equation for A step by step:
$\displaystyle H = \frac {\frac{A * 0.001221}{5} - 0.16}{0.0062}$
$\displaystyle H * 0.0062 = \frac{A * 0.001221}{5} - 0.16$
$\displaystyle H * 0.0062 + 0.16 = \frac{A * 0.001221}{5}$
$\displaystyle (H * 0.0062 + 0.16)*5 = A * 0.001221$
$\displaystyle \frac {(H * 0.0062 + 0.16)*5}{0.001221} = A$
Any problems with this?