The vascular function after interventions as revascularization surgeries is checked intraoperatively and qualitatively by observing the blood ow dynamic in the vessel via Indocyanine Green (ICG) Fluorescence Angiography. This state-of-the-art technique does not provide the surgeon with objective information whether the revascularization is sufficient and should be improved by obtaining a quantitative intraoperative optical blood ow measurement. Previous approaches using ICG Fluorescence Angiography show that the blood flow measurement does not match the reference and overestimates the ow. The experiments indicate that the amount of overestimation is linked to the vessels diameter. We have, in previous work, quantified the propagated error on the flow calculation resulting from the error in the measurement of the vessels diameter and length and realized that they cannot be accounted solely for this deviation. The influence of the transit time error is not revealed yet. We propose a model combining the penetration depth of diffusely reflected photons and the flow velocity profile to estimate the error in transit time measurement. The flow is assumed to be laminar. The photons path is obtained from a Monte Carlo simulation. This is used to determine the maximum penetration depth of each diffusely reflected photon and therefore state how the recorded signal is composed of the signals originating from different depths to check the hypothesis that the error is systematically linked to the vessels diameter. A simplified geometry is set as a homogeneous layer structure of vessel wall, blood and vessel wall. The total thickness ranges from 1 mm to 5 mm. The probability density of the depth distribution of the diffusely reflected photons and the parabolic flow profile are convolved to obtain a weighted average of the ow velocity, which is set into relation with the mean ow velocity. The results show a clear dependency of the error in transit time measurement on the vessels diameter which complies qualitatively with literature and confirms the hypothesis.