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Pulse transit time (PTT) has been a promising non-invasive and cuff-less method to measure blood pressure (BP). However, to achieve acceptable accuracy, subject specific and frequent calibration is required to model the complex non-Iinear dynamic between BP and PTT, limiting the practicality of PTT-based methods. The BP-PTT relationship is often modelled by the Moens-Korteweg (M-K) equation. Previous studies have found that the M-K equation is only able to model high frequency variations of BP, with low frequency variations modulated by the vasomotor tone, which is regulated by the sympathetic nervous system (SNS). In this paper we present a novel calibration procedure that accounts for the influence of the SNS on the BP-PTT relationship. This is achieved by identifying the state of the SNS during calibration based on heart rate (HR), respiratory rate (RR), and PTT itself. The performance of the proposed procedure has been tested on 10 subjects over a period of 4 weeks. The results showed that the measurement differences were 1.04 mmHg ± 6.88 mmHg, and -2.16mmHg ± 6.60 mmHg for systolic and diastolic blood pressure, respectively. Furthermore, the proposed procedure was found to make a significant improvement of measurement precision when compared to the previous one-point calibration method, demonstrating the potential of the new procedure for accurate long-term BP tracking.