Now you can incrementally lower the main timings, for ex 16-19-19-19. Keep in mind that the formula is tRP+tRAS=tRC lets say 14+28=42. If you set tRAS to 38 and tRC to 42, the memory controller will "autocorrect" for the higher value, in this case tRC 42 becomes tRC 52 (14+38). Same goes if tRAS is 21 and tRC 42 => tRAS will become 28 to accommodate the tRC 42.
Also, tRCDRD + tRTP defines the lowest value for tRAS, in your case 20+8=28 is min for tRAS. But if you don't follow the above formula tRAS will get "autocorrected"
As a general rule, tRTP should be half of tWR, but plenty of people don't follow that rule without issues.