To calculate the focal length of an optical element, the two main factors are the refraction index and the shape of the lens.
For a cylindrical lens with one optic active side (that is, one domed or bowled side), we can ignore the whole bottom cylinder and just take into account the top dome. The shape of the dome is determined by the radius of the circle that created it.
Thin, single sided lenses
For a thin, single-sided lens the rather complicated Gullstrand’s formula to calculate the focal length of lenses becomes rather simple:
$f = \frac {r}{(n-1)}$ for the bend facing the object
$f = \frac {r}{(1-n)}= \frac {-r}{(n-1)}$ for mounting it in reverse.
A Polymer database did give a refraction index of PLA as $n=1.465$.
Thick, single sided lenses
For a thick lens with a total thickness of $d$ and one active side, we solve first for the one active side, and then insert:
$$f_1=\frac{r}{n} \land \frac {-r}{(1-n)} ; f_2=\infty ; P_i = \frac 1 {f_i}$$
$$P=P_1+P_2 -P_1P_2\frac d n ; P_2\to0$$
$$f=\frac{1}{P_1}=f_1$$
As long as one side of our lense stays flat, the thickness of the lense is mathematically not relevant (save for increasing dispersion).