It doesn't matter how you set the pivot line as long as you have 25% of the area ahead of the pivot and 75% behind. If you pivot along the 25% chord line, which is a swept line, then your pivot axis joiner rod obviously needs to enter into the fuselage at the same angle. In this case because you'd be using the 25% CHORD LINE that runs along the span the MAC calculation isn't needed and doesn't mean anything. You're working with a whole other method for setting the pivot to get the 1:3 area ratio which doesn't need the MAC location for this. Having said this since it's a simple swept shape the 25% chord line will pass through the 25% MAC line as well. It has to since the MAC is just one more point along the span.
The reason why the straight across line enters the fuselage so much farther back is due to the sweep angle of the surface shape moving the MAC location back so far. There's still 25% of the total surface area ahead of the pivot axis with this method. Just the pivot axis changes because the pivoting is occurring along a different direction.
I would not have even mentioned the idea of a swept hinge line other than because your first part of the sketch showed a line running at what looked mistakenly like a spanwise 25% chord line. I was just correcting the line to show what the proper 25% chord line should look like in case you were considering using a swept hinge line.
And you're right that the straight across pivot method is far more common. And since you've now added that this is a scale model clearly it's the way you should proceed.
Sorry if my added line on the first part caused you any loss of sleep....
