This is a long post, but I promise its worth reading till the end. 
There are two effects acting on the chain causing its actual length to be different from the calculated length, which shouldn’t be mixed up:
1- chain stretch due to the tension force. The chain is an elastic body like any material on earth and will act like a spring, stretching when tension is applied to it.
2- chain sag due its own weight. Like any chain or rope, the weight per unit length causes a sag (engineers call the resulting curved shape under its own weight a ‘catenary curve’). Like the cables holding up a suspension bridge. Or a rope bridge between two mountains that sags in the middle under its own weight.
Chain stretch is probably most easily available directly from the spec sheet, or can be determined experimentally. Hang a chain vertically and measure the increase in chain length as weights are added to its end. This will give a ‘k value’ a.k.a. stiffness coefficient in N/m (lbf/in). Then, when the chain is on the Maslow, by estimating the force based on the position, we can use 1/k to calculate the stretch. The only issue I see is that while tension force due to sled weight is easy to estimate at any position (like @pillageTHENburn anf @Tommy show above), tension due to cutting forces is not easy to estimate, yet cutting forces are significant.
Chain sag due to its own weight is calculatable quite easily (noting that the chain is not horizontal most of the time, so only the component of it’s weight acting perpendicular to the chain’s orientation will cause sag). Heer’s the most simplified explanation of the catenary curve I could find:
https://www.spaceagecontrol.com/calccabl.htm