Distance to leave between drilled holes in plaster
For posterity, I'm copying my comments into an answer... hopefully this can help someone in a similar spot in the future. (Though generally speaking, hitting a stud instead of using drywall/plaster anchors is infinitely preferable.)

I can imagine a few possibilities: 

If you can flip the shelf standards, you might get a totally different spacing. (Granted, they might only work one way up.) 

Next best would be to raise everything by 2 inches. (That's totally anecdotal, btw... I have no evidence that 2 inches is the right number, although @Jack seems to agree.) 

If you can't change height, I'd opt for toggle bolts, with the toggle arms vertical. 

If you were to do shelving like a hack, you'd just angle both of the standards the same way and it would probably work out.