Increasing Gravity

At $t=0$ on the planet Gravitus Increasicus, a projectile is fired with speed $V_0$ at an angle $\theta$ above the horizontal. This planet is a strange one, in that the acceleration due to gravity increases linearly with time, starting with a value of zero when the projectile is fired. In other words, $g(t) = \beta t,$ where $\beta$ is a given constant.

What should $\theta$ be so that this horizonal distance is maximum?

If the answer can be written as $\theta=\tan^{-1}x,$ find the value of $x.$