Suction in retrograde-intrarenal surgery (RIRS) has been the focus of ongoing innovation. Expanding ranges of flexible vacuum-assisted ureteric access sheaths (FV-UAS) combined with single-use digital flexible ureteroscopes (SU-DFU) produce many potential combinations. This study aims to evaluate the efficacy of these combinations in clearing stone fragments. Three different models of flexible vacuum-assisted ureteric access sheaths were tested: Clear Petra (8.5/9.5Fr, 10-12Fr, 12-14Fr), YiGao (10-12Fr, 12-14Fr) and Innovex (10-12Fr, 12-14Fr). A total of 6 ratio of endoscope-to-sheath diameters (RESDs) (0.53, 0.63, 0.74, 0.79, 0.88 and 0.95) were assessed by using three different single-use digital flexible ureteroscopes (6.3Fr Hugemed, 7.5Fr Pusen, 9.5Fr Lithovue). Two techniques, continuous in-scope suction and pull-out with suction were tested across 5 dust size ranges (63-125, 125-250, 250-500 µm and 0.5-1 mm, 1-2 mm). A combination resulting in a ratio of endoscope-to-sheath diameter of 0.53 offered the best range, with up to 2 mm fragments aspirated with equal success by either method. Maximal stone clearance rates at 10.2 g/min were achieved with the combination of a 7.5Fr Pusen with 12-14F flexible vacuum-assisted ureteric access sheath, with a ratio of endoscope-to-sheath diameter of 0.63 using continuous in-scope suction for fragments 125-250 µm. Continuous in-scope suction emerges as the dominant technique for smaller stone fragments and pull-out with suction as dominant or equivalent for greater fragment sizes. The ratio of endoscope-to-sheath diameter is not the only factor critical to improving stone-free rates. Technique should be adapted according to estimated stone fragment size.