you're making it more complicated than it is. looky:
conc. HCl (36%) is 6N. you want 2N? dilute it to a third: 1 part HCl, 2 part water.
maybe use 1.2 part HCl since you only have 31% (-20%). done.
can be done in your head in 20 seconds, has been discussed ad nauseum.
is this exact? no way. but if you need analytical exactness, you can't use
hw store HCl anyway. if you want to exactly neutralise you need an indicator
anyway. so there...