How to calculate the lethal amount of current for given amount of output voltage?
The formula is very basic Ohm's law:
$$I=\dfrac{V}{R}$$
or
$$V=I\cdot R$$
Anyway, the 10mA isn't lethal, a current more than 30mA through the heart may cause permanent damage to heart or infarct.
There is no specific lethal voltage and current, only the recommendation of the max. current that doesn't cause permanent damage. It depends on size of the body, hydration, ..etc.. on what is the body impedance. It also matters how the body is connected to the electrical circuit: hands-foot, head-foot, ... it also differs if you hold the wire with left hand or with right hand.
Generally any DC voltage below 48V is to be considered safe voltage, or AC voltage below 34V.