This method is also referred to as 15-5 distillation, because of 15 theoretical plates and a reflux ratio of 5 used in the distillation. A simulated distillation method described in ASTM D2887 may also be used to obtain TBP data for crude oils.
TBP, ASTM, and EFV Compared | FSC 432: Petroleum Refining
Distillation 15-5 method - Big Chemical Encyclopedia (chempedia.info)
The cut temperature is taken from the probe is placed on the top of the jacketed column connected over the Distilling Flask. You can refer to the picture here. https://brinstrument.com/blog/crude-oil-distillation/true-boiling-point-distillation-astm-d2892-distillation-of-crude-oil/