Calculating Port Length with PVC Elbow?

If you want an accurate way to figure it out, try this.  Take 2 pieces of pvc 4" long each,  fill them with shipping peanuts, pour the peanuts in a bag then insert the 2 pieces of pvc into the elbow, pour the peanuts back into the pvc.  Measure from the top of the peanuts to the top of the tube and this will give you the length of the elbow.

Depending on the size of the pvc you may have to use something smaller than a peanut.  Maybe bb's out of a bean bag chair.

A regular steel tape measure is very accurate;  it will bend through the elbow and you can play it back and forth a bit to find center. 

First, construct the port with the desired length of the first straight leg, and attach the elbow.  The first leg is the part that opens to the front of the enclosure.  For the second leg, insert a length of straight tube that you know will be longer than what is desired, as it will be cut after you measure.  Don't glue the PVC at this time, just insert fully.  Use the tape measure to find the length of this port, then cut off the excess inches from the second leg to the desired length.  Match the piece lengths of the second port to the one that you measured.

Note:  paint the inside of the PVC tube that will show on the finished box.  Do this before assembly and glue.  Placing the business end of a shop vacuum at the other end of the tube will pull the spray paint through and give a drip-free finish.