The operating lengths L1 and L2 have to stay between Ln and L0.
DIN standard requires to respect a minimal space between the coils starting from the value of Sa: Ln = Lc + Sa with Sa = N (0.0015 D2/d + 0.1d).
In case of a dynamic application, when N > 104, Sa is multiplied by 1.5.
IST (Institute of Spring Technology) proposes another approach by considering a percentage of the theoretical maximum travel until the length at solid: Ln = Lc + 0.15 (L0 - Lc).
In some cases where no precision on the maximum effort is required, one can not consider these limitations. In these cases there is simply Ln = Lc. Be careful however, in practice the spring generally reaches its limit at a length slightly higher than Lc.