I believe the 70 came from previous studies where LDL was lowered and 70 was found to be the level Docs should aim to get patients down to. Those studies may have been with other drugs.
70 comes from trials like TNT and REVERSAL. FWIW.
Note in all of this - I am not saying that LDL doesn't matter. I believe it does. However it is not likely to be the primary MOA for the efficacy of statins.