Found this on a Google search for 'Mintra Level'. The last sentence seems to give the sort of advice you are looking for. I well remember struggling with Tephigrams in met lectures in the late 60s and Mintra levels used to come into the preflight met briefs in those days and was indeed used. A Master Pilot QFI went off one day in a JP and drew a set of male 'tackle' in the sky overhead Cranwell. There couldn't have been much wind that day because his artwork 'hung' around for quite a while much to everybodies' mirth. Anyway, I digress........
'' To aid the forecasting of condensation trails emitted (or not) from high-flying aircraft, a line marking the critical temperatures (altitude dependent), above which trails are not possible, is marked on a tephigram (q.v.). The values are approximately -24degC at 1000 hPa (i.e. roughly sea-level), -39degC at 250 hPa (34000ft / 10.4 km) and about -45degC at 130 hPa (50000feet/15km). Using the MINTRA line (as it has come to be called - based on experiments by JK Bannon during World War II with the piston-engined Spitfire), a forecaster will mark two further lines on a tephigram: MINTRA minus 11degC (A) and MINTRA minus 14degC (B). If the ambient temperature (from the tephigram air temperature plot) lies between (A) and (B), then short, non-persistent trails are possible. If colder than (B), then long, persistent trails should be expected. However, some note should be paid to the relative humidity - high values will tip the balance to trailing (or longer/persistent trails.), even with air temperatures warmer than (A); ultra-low rh% will reduce the risk of condensation trails - the design of engines will have an effect as well. In broad terms, warm Tropical Maritime airmasses with a high but cold tropopause will result in a good deal of trailing, whilst cold, polar air-masses with a low, relatively warm tropopause will seldom give rise to significant aircraft trails.''
Cheers,
mcdhu