According to a review by Legge & Bigelow the arc or degrees of visual angle ($\alpha$) is,
\alpha = 57.3 \times S/D,
where S is height of object and D is distance to object.  $S/D$ is the small angles approximation of $2 \times arctan(S/2D)$ which follows form geometry.
Image 1: the equation comes straight from trigonometric definitions.
Since humans continuously scan the page with the fovea, small angles assumption is a good approximation for entire pages. The factor $57.3$ is simply $180/\pi$ or conversion form radians to angles.
So given that we do not know the nature of $tg$, then its hard to say. Looks a bit fishy, but if tg is some sort of inversion of S then its plausible.
 G. Legge & C Bigelow. 2011. Jornal of vision. Does print size matter for reading? A review of findings from vision science and typography.