I usually choose pure water when I dilute nucleic acid solutions for spectrometry. This should be done when A260 > 1.0. If the undiluted solution has an absorption of 2.0 for example you could prepare a 1 to 10 dilution (1 microliter undiluted solution plus 9 microliters of water). Mix the dilution by pipetting up and down a few times. Then use this dilution for the Nanodrop measurement.