How do I get the actual member length of cables, for example, after the calculation in RF‑FORM‑FINDING?

This information is available in the Results navigator for members (see Figure 01). The member lengths are displayed with regard to the stressed structural system and the unstressed system. The "stressed length" is obtained from the form-finding under consideration of prestresses.

The recalculation to the "stressed length" can be done manually using Hooke's law:

${\mathrm l}_{\mathrm{unstressed}}\;=\;{\mathrm l}_{\mathrm{stressed}}\;-\;\mathrm{Δl}\\\\\mathrm\sigma\;=\;\mathrm E\;\cdot\;\mathrm\varepsilon\\\frac{\mathrm F}{\mathrm A}\;=\;\mathrm E\;\cdot\;\frac{\mathrm{Δl}}{\mathrm l}\\\mathrm{Δl}\;=\;\frac{\mathrm F\;\cdot\;\mathrm l}{\mathrm E\;\cdot\;\mathrm A}\\\\{\mathrm l}_{\mathrm{unstressed}}\;=\;{\mathrm l}_{\mathrm{stressed}}\;-\;\frac{\mathrm F\;\cdot\;{\mathrm l}_{\mathrm{stressed}}}{\mathrm E\;\cdot\;\mathrm A}\;=\;{\mathrm l}_{\mathrm{stressed}}\;\cdot\;\left(1-\;\frac{\mathrm F\;}{\mathrm E\;\cdot\;\mathrm A}\right)$