offset? 57c on a kouki

hey, i just bought a 97 240sx and i put my old tenzos on it. they are 18x7.5 with the standard +43 offset. i am going to buy the gram light 57c and im getting 18x8.5 in the front and 18x9.5 in the back. i was just wonderin what offset i should use because im not sure how to check. i was thinking +30 all the way around. i measured my old wheels gap from the edge of the wheelwell and i have 1 1/2" up front and 1 3/4" up front. thanks for the help and please dont criticize. im only asking an opinion and ive done my research.