This research assesses the effect of two common assumptions in simplified flame spread models: (i) that the pyrolysis front propagates in one dimension and (ii) that ignition time () and heat release rate per unit area (HRRPUA) values measured at bench scale can be utilised in simplified flame spread models. To that aim, the study introduces a methodology to measure time to ignition, time-dependent burning area, heat release rate per unit area, and ignition temperature for concurrent upward flame spread at intermediate-scale, while examining the feasibility of using these metrics to predict flame spread scenarios. The methodology employs a narrow-spectrum illumination source and thermocouple arrays to precisely track the pyrolysis front progression during flame spread experiment. Measurements were obtained for Poly(methyl methacrylate), and they can serve as reference points for researchers aiming to develop or refine testing protocols or perform sensitivity analyses for assumptions relevant to time to ignition and heat release rate models. Measurements were compared with cone calorimeter data from standard and modified testing protocols in the literature. Post-ignition removal of external heating reduced HRRPUA values, aligning them more closely with intermediate-scale experiments, while spatial heat flux distribution changed the time to ignition towards the values that were measured in intermediate-scale. The impact of the solid ignition parameters on the flame spread rate was evaluated individually using a simplified flame spread model. The proposed methodology provides a means to refine extrapolation methods for solid ignition parameters.