There are a few methods used to calculate magnification, but here is the simplest, which works quite well.
1.)Estimate the ratio of subject length to width or height of field.
Example = at 1:1, bee length is 1/4 width of field.
2.) Crop until subject (bee) fills 1/2 width of field. You have increased length of bee by 2x, and height by 2x.
2 x 2 = 4:1 magnification = 4x life-size.
-or-
3.) Crop tightly, until subject (bee) completely fills width of field. You have increased length of bee by 4x, and height by 4x.
4 x 4 = 16:1 magnification = 16x life-size.