Is the ligation ratio correct in the pGIPZ empty vector product insert where it says 250ng of pGIPZ to 7.4ng of insert?
Yes, it is correct. You must have more of the pGIPZ vector because ng is a unit of weight. Since the pGIPZ vector is ~11 kb and the insert is ~0.35 kb, you will have about 31 times less vector than insert per ng. You must increase the amount of vector used to equal the number of pieces of insert used.