Your formula doesn't seem to take into account two important factors: layer height and average speed.
Do the math differently:
voumetric flow rate [mm^3/s] = layer height [mm] * line width [mm] * speed [mm/s]
For example, 0.2 * 0.45 * 70 = 6.3 mm^3/s
which is realistic, the extruder on the Ender 3 cannot go much faster than that reliably.
PLA weighs 1.24 g/...
Your math looks correct, and is also a good approximation for what I've seen in the first few weeks with my own Ender 3.
Another way to calculate (to check yourself) is to calculate the volume extruded (nozzle area times extrusion percentage times print speed -- be sure you convert everything to the same units!) in a given second, multiply by the density of ...
Bed heating is not required when printing in PLA. But if it is an option and you would like to heat the bed, I would not go above 70/80 °C for PLA.
As far as fan speed, I would go 90-100 % especially if the bed is heated.