The SET STEPS PER in the settings tab will be your best option but you due to the fact you don't fully know the pitch of the screw and if it's cheap lead screw, which it will be, then there could be error between spec's and reality.!! The SET STEPS PER gets round this because it measures actual movement BUT YOU MUST MEASURE ACCURATELY.

Best easyist way is to get the longest vernier gauge you can find, longer the measurement the more accurate the machine will be. Clamp the end to the bed with a block of wood or something in between but still so the scale can slide then open them up wide as will go and zero them out, then bring the Z axis done so when you type movement amount in SET STEPS PER it push's them towards closed position. This will give you most accurate reading with out user error creeping in.

I recommend you do it twice, the first time with a small movement just in case your miles off with the steps per in motor tuning,(Dont forget to save each time). Do this for all Axis, this will get you very close, then do another pritty much the full length of your verniers capabilty.

Hopefully this will get you back on track.!!