Context. The AMS-02 and HELIX experiments should provide soon 10 Be/ 9 Be cosmic-ray data of unprecedented precision. Aims. We propose an analytical formula to quickly and accurately determine L from these data. Methods. Our formula is validated against the full calculation performed with the propagation code usine. We compare the constraints on L set by Be/B and 10 Be/ 9 Be, relying on updated sets of production cross sections.Results. The best-fit L from AMS-02 Be/B data is shifted from 5 kpc to 3.8 kpc when using the updated cross sections. We obtain consistent results from the Be/B analysis with usine, L = 3.8 +2.8 −1.6 kpc (data and cross-section uncertainties), and from the analysis of 10 Be/ 9 Be data with the simplified formula, L = 4.7 ± 0.6 (data uncertainties) ±2 (cross-section uncertainties) kpc. The analytical formula indicates that improvements on L brought by future data will be limited by production cross-section uncertainties, unless either 10 Be/ 9 Be measurements are extended up to several tens of GeV/n or nuclear data for the production of 10 Be and 9 Be are improved; new data for the production cross section of 16 O into Be isotopes above a few GeV/n are especially desired.