If the calculation via the Assumed Temp method gives you a target N1 of 98.3% and the fixed derate gives you a target N1 of 98.3%, they give EXACTLY the same thrust and EXACTLY the same margin!
However, assumptions and procedures differ, so your margins will vary. For example:
Your company procedure is to use standard derates. You go into the book and find that at Derate 1 for your conditions, N1 is 98.3% and the max TOGW allowed is 300.0. Derate 2 has an N1 of 96.8% and a max TOGW of 275.0.
If your TOGW is 275.0, you can use Derate 2 with the minimum legal margin, or Derate 1 with more margin. If your TOGW is 275.1, you must use Derate 1, and your margins will be significantly higher than if your TOGW was 300.0.
Now say your company procedure is to use a manual or computer-assisted Assumed Temp Reduced thrust. If the margins used by the company are the same as those used in the max TOGW calculation of the derate tables, and your TOGW is 275.0, it will give you an N1 of 96.8%. If your TOGW is 275.1, it might give you 96.9%.
If the company uses other margins, all bets are off.