How much can I reuse paraffin to wax to my chain?
Paraffin wax is not a single chemical, rather it is a mix of alkanes with the formula CnH2n+2, having between 20 and 40 Carbon atoms. Commercially available Paraffin wax may also contain additives to raise melting point (normally 37 °C, 99 °F) or reduce brittleness (often stearic acid as found in tallow).

It's possible that when you immerse your chain, the longer alkanes are more likely to be retained in the chain. The stearic acid molecule (C17H35CO2H) is only 18 Carbon atoms long, but the O and OH groups may increase the chances of it adhering to a surface.

Alkane cracking might also be occurring, catalysed by some impurity in the pan or chain, breaking the longer molecules up into shorter alkanes. Paraffin's density should be about 0.9g per ml. Alkanes from C5-C16 (inc octane, decane) are liquid at 20 °C with density 0.63 to 0.77 g/ml. The shortest Alkanes are gases: Methane, Ethane, Propane, Butane. With an accurate scale you may be able to detect a difference in volume between your used wax and the same volume of fresh wax.