Change this:
setElementHealth(gracz,tostring(currentHealth - 0.002))
To this:
setElementHealth(gracz,tonumber(currentHealth - 0.002))
Also you must define who is gracz.
I found something that might be useful.
How to calculate:
http://wps.prenhall.com/wps/media/objec ... /E03.2.pdf
Online calculator:
http://www.smartconversion.com/unit_cal ... lator.aspx