You could easily say to a soundman that you'd prefer that the upper mids and highs of your sound be handled by the mic, and that you'd like the DI signal to handle the lows.
As for the math and numbers, it's a tough call because a specific number won't work equally well in all rooms - the low end portion will be quite variable, but you could ask for the mic signal to have a boost at 4kHz , 500kHz or 800kz as a start, but you need to know and have experience with what those frequencies bring to the table sonically in terms of a bass tone. This is where some self-education and exploration of the audio spectrum will serve you well.
There's also the point of finding a soundman at a club who will do all this for you, as most bass soundchecks last no more than 10 seconds, with no prior consultation