Front rim back spacing question

[GNBRETT]

I have a set of Bogart front rims (15x3.5). I am in the process of ordering custom (15x6) Bogart front rims and Bogart asked me what back spacing I wanted. I am ordering new rims to drive on the street. Can't afford to buy a whole new set otherwise I would get the street lites from Billet Specialties. My fronts with drag tires will be for sale once these come in. It takes 3-4 weeks they say.

Can anyone tell me what the proper back spacing should be for a set of 15x6 front rims? Thanx...