I have been using a computer software program called "G-tune". The program is no longer sold, but has been replaced with another called StroboSoft.
G-Tune seems to work very well, although I have not had a second opinion on my tuning efforts (accordions only, so far).
For fine tuning accordion reeds, I have a hardened (heat to cherry red and quench in water) piece of .030" music wire pressed into a wooden dowel for a handle. This is used to "scratch" the reed to move/remove small amounts of material. Is this an acceptable method with brass reeds?