Here’s a useful tip for those that struggle with larger curvatures (like found with ellipses) or other lines/shapes that change little or slowly over a relatively larger distance. It is often far more difficult for artists to detect the problems with slow or subtly changing lines (like spotting the differences between ellipses A and B in blue) as opposed to lines or shapes that have greater and faster changes over that same distance (seen in green). However, in many cases we can change the slow and subtle (blue) to quicker and greater (green) by changing our perspective, or orientation to the drawing or painting surface.
Try looking at a problematic slow or subtly changing line or geometric figure at a near parallel angle (as shown) to condense how it is falling on the retina. This way, unwanted problems/disparities/deviations may appear far, far more apparent–yielding the opportunity for a more effective assessment prior to making changes. Remember that there may be some perspective distortions that one will have to contend with from viewing a line or figure severe perspective—but the information yielded should far outweigh the inconvenience of dealing with the transient distortions.
Happy Painting!