The primer concentration has a major effect on the performance of a qPCR and must thus be optimised thoroughly. If the concentration is too low or too high, the calculation of the Cq-values may result incorrect, and the danger of primer-dimers increases. A good guess is to start with a primer concentration of 200-300 nM and to also include higher and lower concentrations in a test assay.

Mikeska, T., & Dobrovic, A. (2009). Validation of a primer optimisation matrix to improve the performance of reverse transcription – quantitative real-time PCR assays. BMC Research Notes, 2, 112.