Compiled by Jonathan Dsilva.

Noting the sequence of descriptions -
1852 - rigidistria, Guenee, described from a female specimen from Central India. mentions the FW medial line "bordered internally by melted white-lilac" (translated)
1858 - calefasciens, Walker described from specimens from Sylhet(?) & Ceylon, mentions  the FW medial line "broadly bordered by whitish lilac on the inner side" 
1884, Lep. of Ceylon, Moore Vol 3  - calefasciens,  describes the medial line as - "straight inwardly-oblique line, which has a broadly diffused purplish-lilacine white inner border in the male, and is broadly bordered on both sides with lilacine-white in the female" . The plate 168/3 is supposedly of a male. but this has lilacine white diffusion on both sides of the medial line. Probably, he has got the descriptions reversed.
1894, MOI, Hampson Vol 3, combines calefasciens under rigidistria, mentions "medial whitish band with a dark line on its outer edge", i.e. as above. TL includes Sylhet & Ceylon
1989, Holloway, MOB, calefaciens , in comparison to Buzara mentions the medial line "usually has a paler zone grading away basad rather than one distad". Also "There is also a diagnostic but small pale dot in the cell basal to the straight line.". In the taxonomic note, Holloway refers to "rigidistria complex" mentioned in Holloway, J.D. and Miller, S.E., 2003. The image shown is of a female, but without much lilac-white medial diffusion. He also mentions - "This dissection has also indicated that rigidistria Guenée is restricted to the Indian Peninsula and Sri Lanka". "The concept of calefaciens presented here is that of the Doubleday (?Silhet) syntype, a male (slide 18381); the Sri Lankan syntype has not been located.". The TL mentioned here does not include S. India or Sri Lanka
2013, MOT, Vol3 pt2, Kononenko & Pinratana, has both calefasciens & rigidistria, & the TL for both includes India, Bangladesh & Sri Lanka. The image of rigidistria has pale diffusion on both sides of FW medial line, while calefasciens does not.
Inaturalist - rigidistria, has images with diffusion, from S.India as well as NE India. calefasciens has images without diffusion from S.India & NE India. also has a few images with diffusion from S. India.
In conclusion - if Moore is correct, then the presence of diffusion on one or both sides may be used to distinguish male from female
If Holloway is right, then all images from S.India (including peninsular India) should be rigidistria, while those from NE India, should be calefasciens. But then the presence or absence of diffusion would not be a diagnostic feature.
If the separation based on medial diffusion is correct, then both species ranges overlap.
There is no mention in any of the above references to distinguish the two species by the medial line diffusion or from HW patterns. Need to check if there are any such references.