You can try 1:500 dilution of 5.0mg/ml stock. You can incubate for 3-4 minutes and optimize time accordingly. Permeablize the cells prior to DAPI staining and do the DAPI staining in a buffer without any detergent such as IX PBS without (Triton-X100).

Then wash with with 1XPBS for 5-10 minutes and mount sample.

Other way of staining with DAPI is to use a mount media with DAPI such as Vectashield with DAPI.

Thnkuuu

I regularly use it 1:2000 with 5 mg/ml stock solution and incubate for 1 min.  Good lucky.