for calculation (dunno if it is the same for digicams) using film camera is this
3T = 3 diopter
4T = 4 diopter
hence if using 3T
0.85 X 3 around 0.3
hence you get 0.3 life size reproduction
when you ger 1:1 life size reproduction, it means the size of the object on the film will be the same size as the real subject.