Who says that adding a sentinel won''t change the angle of the rode to the anchor? It will ALWAYS change the angle of the rode to the anchor, unless its attached right at the bow of the boat.
The heavier the sentinel and further down you place the sentinel the more effect it has. Obviously in some circumstances it additive effect is minimal but it will always have some effect.
Did you look at the web site I listed or go into the Excell Spreadsheet? It has diagrams and a write up on exactly that and the Excell spreadsheet program even has a diagram/picture that shows exactly how the anchor rode will look depending on what variables you put in.