This paper proposes the use of the non-uniform probability distribution function to improve the performance of passive UHF RFID systems with several readers. In a previous work, the authors proposed GDRA, a mechanism based on the NFRA protocol where the readers used a truncated geometric distribution function to select a contention slot, instead of the typical uniform distribution function. Results showed that GDRA increased the RFID system performance but, this improvement could be higher if the optimal non-uniform probability distribution function is used. In This was not possible due to the inability of readers to calculate the number of (interfering) neighboring readers, a necessary parameter to apply it. This work solves this issue, using a mechanism where readers calculate the average number of neighboring readers through a simple formulation extracted from graph theory unit disk applied to wireless sensor networks.