How can I improve manual-focus accuracy with a split-prism screen at wide apertures and close distances?

Yes—this situation is inherently difficult. A wide aperture combined with close focusing gives very shallow depth of field, so even tiny focusing errors will show up.

A useful split-prism technique is to find a straight line in the scene, place it in the center split-prism area, then rotate the camera slightly off level while focusing. The angled line often makes it easier to see when the two halves align perfectly. Once focus is set, recompose and shoot.

Beyond technique, the main limitation is depth of field: at close distances and large apertures, the in-focus zone is extremely thin. If you need more margin for error, stop the lens down to a smaller aperture. For very close or macro-style work, using a much narrower aperture is common, even though diffraction can become a tradeoff.

So the practical answer is: use the split prism with a strong straight edge if possible, try the slight camera-tilt method, and consider stopping down when shooting close-up if critical sharpness matters most.