Values between log scales in a graph
suppose my value is 120mA, where should I place my point

If you split part of the vertical scale into decibel ratios you can get close to a line that corresponds with 120 mA forward current: -

So, I've made 100 mA = 0 dB and I've made 200 mA = 6 dB. Of course, as we know, 200 is 6 dB higher in signal magnitude than 100 mA. Strictly speaking a 2:1 ratio is 6.02 dB but that 0.02 dB error is miniscule given the graphs physical resolution.
120 mA is 1.58 dB above 100 mA so, the 120 mA line will be roughly where the +1.5 dB blue line is. The corresponding forward voltage is around 2.8 volts.