To start, you only need a 90 degree elbow for the light trap.

This gets a bit trickier b/c you have to know intake vs. exhaust temperatures to do the calculation. Further, if you are drawing from outside your intake temperatures are subject to change by time of day and season.

Is this the old fan from your previous setup? If so, I would start with that and a fan controller to see how far off you are - if at all. I recall reading somewhere that a good rule of thumb is to increase your fan by 100 CFM for every 10 degrees you need to lower the temp. You should try to verify this as it is hearsay. Also note that I have not had to deal with your situation directly so my comments are based on related experience and reading...

I'm going to assume you have the old fan. So, hook it all up with the existing fan. Make sure that the thermometer is not in direct light! It should be in the shade about 1/2 up the wall. Let the lights run for 1 hour without the fan. Take a temperature reading. Next, run the fan on full and take temp readings at 1, 2, and 3 hours. At 3 hours you will know how far off you are. If your temps are 10 degrees too high then get a fan with at least 100 additional CFMs. If it is 10 degrees too low then dial back the fan. Also consider getting a fan speed controller that is temperature activated, if you don't already have one.

Always overestimate your fan needs. You can always dial it back. If you are drawing air from an external source and if it is cold where you are, consider the implications for warmer weather.