There is a very useful article on muhūrta by Pt. Sanjay Rath. In that article, he distinguishes muhūrta as a time measurement unit (defined, for instance, in Śrīmad-Bhāgavatam, Canto 3, beginning of chapter 11) and muhūrta as a daily astrological phenomenon. While the former’s duration is fixed, the latter’s duration varies depending on sunrise and sunset, and is equal to 1/15th part of the period between the two, distinguishing daytime and nighttime muhūrtas.
The same view is expressed in “Hindu Electional Astrology”, section 7.6.5, authored by V. K. Shridhar (2002). In that profound work, Abhijit muhūrta is also described as the middle 8th muhūrta of the daytime. We elaborate a bit more on the topic of muhūrtas in a related article on Brahma muhūrta.
Chakra Darshana implements the varying approach suggested in the two references above. An alternative would be to calculate Abhijit muhūrta using a fixed duration of 48 minutes (i.e., 24 minutes on either side of local noon). However, we do not have any references supporting that view.