We do alot of things by eye. If you make it level it looks off, so we will just eyeball it to make it look best.
We also will measure distance from a certain wall or point and keep the distance the same instead of making it "level" so then our new constuction is "equally unlevel" as the walls or point of reference.

It is kinda hard to explain, so hopefully it makes sense to you

-renee

When we did our bathroom we started in the middle and went from each side. Our bathroom was not straight or level. We did the blocks 12 inch 6 sided white and black. We felt when we finished that bathroom and we were still married we were so proud of ourselves. Nothing was straight on the sides and we put bead-board half up and wall paper that looks like tin but you paint it on the top. Because we didn't purchase the toilet and sink in our town none of the plumbers would install them.. Finally found one. Good luck..

Our main house has straight level walls and floors. But the addition is anything but. It is amazing how construction standards slipped between 1850 and 1900.

Avoid straight lines. If you need them, make them on the diagonal.

Straighten and level where you can. We leveled the kitchen floor and made a "ramp" to the family room which is not level. It drops 1 inch over the 24 inches beside the cabinet, but that is not noticeable. A level kitchen is more important, and flat was really important for our click together cork flooring.

When we had dormers added to part of the roof, the GC raised the worst dropped corner of the roof beyond the dormer and added a 12 inch trim board to disguise the fact that, unlike the rest of the walls, the top 6 inches was wood, not stone.

Inside that room, we opted for one colour paint over walls and ceiling to deemphasize the wonky joints where wall meets ceiling.

With old houses it's "Plumb, Square and Level......pick two."