i think logic would suggest there isn't actually a cutoff point and the mixture continues lean out as the engine gets warmer. The incremental change in resistance drops as the temperature rises until it hits optimum at operating temp, which also explains why it would lean the mixture still further when engine temp rises past normal.

i had an interesting few weeks with my 300E in this area when I overhauled the cooling system after the visco fan clutch failed. Replacing the radiator brought operating temp down from 87 deg to below 80 deg, which meant the car was always running rich thinking it was in still in warm up cycle. Really noticeable going up the mountains, the car just wasn't happy pulling up a hill.

Turned out the PO had compensated for poor radiator performance by replacing the thermostat with one that opened at a lower temperature. Replacing it with the OEM spec unit sorted everything out

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1987 300e manual 250,000 km (sold)