how to calculate self discharg %/month for lithiam ion battery?
If you don't believe the vendor's data sheet, then you don't calculate it, you measure it. Measure drop in terminal voltage month on month, and relate that approximately to capacity via the data sheet for the battery.

It varies with temperature and vendor quality. If it's not a new cell (thanks to Brian's comment) then it will vary with the use/abuse it's had during its life, depth of discharge, charge termination voltage and current, number of cycles, charge and discharge temperatures.