Need help for calibrating my espresso - Extraction time for 16g

Hi guys, so I have recently bought a Hibrew H10b and I'm trying to calibrate my espresso. It fits 16g (the problem with 18g is that it touches the part where the water comes out). What should be the extraction time to aim for? I know it is 25-30s for 18g, but for 16g?

Right now I'm getting 40g of espresso out after 30s (5s of pre-infusion), and it comes out a little "acidic", I don't know the word in spanish we say "acido"

So yeah, pretty much I would like to know what time should I aim for considering the 5s of pre-infusion and that the input is 16g of coffee and its coming out a little bit acidic.

Thanks in advance!