Renato,
How are you determining Zone I? Do you have a densitometer, (if so, are you developing in pyro, which will give false readings)? What values are you using for Zone I?
Are you using a printing method, e.g. "first recognizable step above maximum black"? Are you sure your meter reads well in the predominantly blue light of open shade?
Also, you may want to test for highlights (Zone VIII density) soon. If you are overdeveloping for "Normal," your film speed will be proportionately higher (1/3 to 1 stop depending).
There are many things you need to tell us/take into consideration before you can nail down "Zone I".
I make "Zone Rulers" at different EIs and contact print them to calibrate my film. This gives me the right development for the particular paper I like to use without any densitometer middle step. If you are interested in my method, contact me off-forum and I'll e-mail you the procedure.
Best,
Doremus Scudder
www.DoremusScudder.com