Depends on the cord your using.
Some shock cord stretches more than others, so its a "test it" kind of solution. What I do is leave the shock cord long (even on the spool), then set up the hammock, hang the uq, adjust the length of the suspension, then trim any excess cord off and knot it. This eliminates any excess weight from the quilt suspension.
Each uq is different in the diy world, so if your designing more quilts, you may need different lengths of s.c. suspension to get the proper fit, depending on weight of quilt, strength of shockcord, length of hammock or quilt, etc.
If you are building more quilts of similar size, weight and length, then you can use this quilts cord length as reference.