My feeling is that there won’t be a huge difference in the nHL-to-eHL for adults depending on whether you use inserts or supra-aural headphones. The reasoning is as follows: there are two considerations when deriving the nHL-to-eHL correction. The first is the age-related transducer correction (this is a calibration correction and is applied for infants under 6-months of age and is much greater with insert phones due to small ear canals in infants and less with the supra-aural headphones due to the larger overall enclosed volume). The second consideration is the actual underlying difference in ASSR threshold and behavioural threshold (and this effect should be much the same irrespective of the transducer type).
So here the first of these two considerations doesn’t apply as we’re talking about patients over 6 months of age while the second one is not dependent on transducer type.
We should also bear in mind that you can still make an accurate determination of normal hearing, reliable PTA/ unreliable PTA etc… without any correction factors at all. You can still make those determinations (at least in the interim, until your inserts are available) in nHL without switching to eHL.
All in all I’m therefore inclined to suggest basing your conclusions/reports on the nHL thresholds where possible, unless needs specifically require eHL (e.g. need to fit a hearing aid) and in that case use the insert values due to lack of a readily available alternative, but rationalised using the above explanation. When your insert transducers are replaced then you can of course revert to normal practice.
The origin of the nHL to eHL correction factors is noted below for insert phones. You will find this information on p140 of the Additional Information Eclipse manual installed on the Eclipse computer (it should be at the following location C:\Program Files (x86)\Interacoustics\Eclipse)