Hi @farhhhan,
concrete as a material is heterogeneous. I have never used ACI but in the BS standards I use, a value of 0.67 f'ck which converts the cubes strength to cylinder strength because the cylinder more closely predicts the strength of the concrete (0.85 is the ratio we use) and an additional factor is used to take into account the size effect since concrete strength varies with size (as you have stated) plus the rate of load since in the real world loading isn't added as quickly in the lab. Very many studies have been carried out and it was proposed to take only about 0.8 of cylinder strength to account for all this which gave 0.67 (0.85 x 0.8). you use 0.85 which relates to my 0.8 so I feel you are ok.