Is there an equation that allows me to convert the resistance of an LDR to lux. For example, when a LDR reaches a certain resistance (which would equate to lux) lights should turn on.
I want my LDR to turn on an LED when lux levels drop below 25 lux
In order to make a lightmeter, each LDR needs to be calibrated separately.
In your case, use another lightmeter for comparison, measure the resistance of the LDR (or the analogRead() output) at 25 lux and set up your if() statement accordingly.
The BH1750 is very cheap and returns the lux value. It is 20% accurate. The lowest value it can detect is 0.11 lux (with a special extended range), it can not detect something below 0.11 lux.
LDRs are not that accurate; you'll have to calibrate and then you should be able to get reasonable results. They're best used for basic dark vs. light detection.
If you want accuracy get a better sensor such as the TSL2591.
LDRs may be very sensitive to temperature too, depending on the type. Photodiodes are generally superior and faster to respond too. Photocurrent in a photodiode is very insensitive to temperature except when extremely dark and is linear with photon flux.