I am working on isolating DNA of samples from a cohort study- I'm getting about 60-100ug yield, and I'm curious if anyone knows of an optimal stock concentration for DNA storage. We have to make a stock of 50ug/ml for Picogreen, and we're going to use it for a methylation assay (which also need 50ug/ml) and for PCR

The more concentrated is the more stable. However, genomic DNA is really stable and should last for years at concentrations between 100-2000 ng/µL (or µg/mL). You should make a stock solution according to your future needs, like 500ng/µL.