Single-layered two-dimensional crystals of dust particles are often observed in dusty plasma experiments and the data on their structure can be obtained by analyzing the images usually taken by charge-cupled device cameras. We give here some formulas for practical purposes of estimating the screening length and the electric charge on a dust particle from the surface density and the radius of such finite two-dimensional dust crystals. The formulas are derived on the basis of our theoretical approach which has been successful in reproducing results of molecular-dynamics simulations on dusty plasmas. An example of application is also shown.