The starting point is a high accuracy volt meter with silver electrode(s). You will need more than that. A friend who has been helping me has been otherwise occupied. And, so have I.
For your use though, 0.1 molar NaBr at 20 deg C will have a vAg of -61.48 mv and a pAg of 11.463
That is, BTW, 10.289 g/l.
So, here is a calibration point for you.