I have noticed that the link I posted 18 months ago is no longer useful. DPA has changed their Web site and no longer offer information at that link. They do provide information on spacing of omni mics in a couple other places on their Web site. The first link I liked is focussed on recording an orchestra, which is a task very similar to field recording!
https://www.dpamicrophones.com/mic-university/classical-orchestra-a-b-stereo
"Channel separation in the A-B Stereo technique is determined by the distance between the microphones. Compromises have to be made so that the orchestra sounds natural with a proper stereo width. Normally the spacing is adjusted between 40 and 60 cm. Some producers favour greater spacings of between 1 and 2.5 meters and sometimes more, but in these cases a hole will start to appear in the middle of the stereo image. This can only be compensated by using a third microphone placed between the two others.
The use of A-B Stereo techniques without support microphones can create an extremely convincing depth in the stereo image and capture a realistic room impression. The sound sources, ie musical instruments and room reflections, are picked up with the correct time alignment relative to the placement of the main stereo pair, which explains why this method is often regarded as the purist’s choice."
The second link I liked is this more technical analysis of mic spacing for a variety of different microphone types, including omni and directional mics. It is a very thorough discussion of stereo mic principles and techniques.
https://www.dpamicrophones.com/mic-university/stereo-recording-techniques-and-setups