Adding my own dim in sketcher

What do you mean you "cannot override it"?  Have you tried just creating the dimensions you want?

Sketcher automatically creates enough dimensions to fully constrain your sketch.  These are 'weak' dimensions which disappear when you create your own dimensions that constrain the same degree of freedom.  Therefore, you can easily tell whether you've fully constrained your sketch: there should be no weak dimensions left (either replace them, or if you want to keep one, RMB and set it to 'Strong').

I'm with you with this struggle.

I think there isn't too much trouble with boxy sketches but when you raise the complexity you also raise issues with what the software considers as a fully constrained sketch.

As Jonathan suggests it is good practice to remove the weak dimensions in order to get a sketch that acts like it should when changing a dimension.

Working with angled lines attached with radii is torture in a sketch.  If possible it works best with radii done outside of the sketch.

I've many times tried creating a specific dimension and the software just doesn't allow it.  Sometimes you can sneak up on the answer by making some of the weak dimensions as strong and then deleting the strong dimension.  If you've placed some dimensions prior to this you might get lucky and the un-needed dimension won't pop up again as it might see the constraints already satisfied.  If you are unlucky the weak dimension will pop up again in the same place.

Perhaps there is an easier way to fully dimension a sketch.  It is aggravating to try placing a locked dimension and it being over ruled by an un-needed weak dimension.