You are going to have to calibrate your setup.
You will need a test tone (like a 1k sine wave) and the owners manual for your interface.
Look in the manual to find out:
1. Are the line inputs on the interface +4 or -10
2. what the converters are calibrated to (line level input = ?dbfs)
It is a lot easier if everything has a setting for unity gain. (the busses on the mixer might)
What you do is follow my instructions from before and set the input trim so the meter on the mixer reads 0dbvu.
Using the pfl on each buss of the mixer, make sure that the level is still 0dbvu all the way through the mixer.
Plug the output of the mixer into the line input of the interface. If there is a unity setting on the interface, set it there and you are done. If there isn't, you will need to know what level in the computer equals a line level signal at the interface. Once you know that, you can set the trim on the interface so your test tone reads that level in the computer.
Now that you have done that, you can leave the trim control on the interface alone forever. You will always adjust the level with the mixer.