134a equivalent to r12 Freon

My van has front and rear air which I converted to r134 from r12. I replaced the dryer, compressor and flushed the whole system with A/C flush. My question is from a chart I downloaded it says my system requires 72 oz of r12. Now that I am using r134a do I still use 72 oz? When looking at equivalences r12 weighs 3.0 oz per oz of r12 verses r134a weighs 2.6 oz per oz of refrigerant. Does this mean that I should add an additional 8 oz of r134a to my 72 oz called for making it 80oz? I arrived at that sum by taking 72 x .04 which equals 21.6 then divided that by 2.6 which comes out to about 8 oz of 134a to equal the same weight as the r12. I hope you understand this question.