Defaults (Voc is the STC value at 25 °C): REC Alpha Pure-R 430 W class, manufacturer datasheet, US print UL EN 6.2025: Pmax 430 W, Vmp 50.5 V, Imp 8.52 A, Voc 59.7 V, Isc 8.91 A. The 80 cells in series (the sheet does not print the layout), ideality 1.0 and the 25 °C cell temperature are our assumptions. The FF to test (0.75) is invented.
Teaching aid, not a design check. FF = Pmax / (Voc x Isc). Ideal FF uses Green's expression and the series-resistance form as read in Javier et al. (APVI conference paper, Table II), valid for normalised Voc above 10 and normalised series resistance below 0.4. All shortfall is put on series resistance, so the resistance shown is an upper bound.