Digital BW, The Print

Ii am pulling out old film cameras to shoot black and white film to scan, then to digitally print. I will primarily be shooting medium format. I live in then land of no sun,Seattle Wa  so use tmax 400 rather than a 100 Asa film. I will be using a good lab to develop the film withou any push or pull. They provide a low level scan with developing for a reasonable price. I will have them do a high Rez drum scan when I have a print made. My golal is to have 20 by 20 prints. Ok, my question... In reading this site it seems like I need to under expose the film. Is this correct? And if so by how much? And just to make sure I get this right I would set the light meter for say 500 Asa for tmax 400 (not set the meter for Asa 300)

Thanks!