How do I find bacteria/ml from a turbidity reading on a Spec20?

Question

I already grew the bacteria and measured the optical density

Answer 1

You should have done a standard curve. First plate samples of different turbidity (that you measure) and then count the colonies growing for each one. From the number of colonies you can deduce the number of bacteria/ml and do the standard curve.

You can find an approximation in the manual of a QIAGEN kit (I think the mini-prep kit) for 2 types of spectrophotometers, but the reading depends also on the type of bacteria, sometimes the strain if there is a mutation affecting size/shape.(You can look for it on the net).
If you plan to use this assay frequently, do a standard curve. If you only need for this time, plate a series of dilutions of your culture and then calculate the #bacteria/ml based on the colonies formed.