Problem is as far as I know there are only IR-Thermometers for condensed matter = no gases. There are
thermal imagers like you see in movies, who may actually also give quantitative amounts and therefore measure gas, but they are up to
3700 € .
But the actual flame itself will be ways too hot anyways, so people just hold the flame far away anyways to get down from ~ 1400 °C to 150 - 250 °C roughly.
You could buy a thermometer up this high (maybe there are some below 20 °C) and just check at what distance your torch lighter will produce this exact heat.
Problem if you would do Changa is:
It will be no homogenous surface, so if a leaf is guided towards the flame, then its surface will be ready for vaporization. But the backside of that leaf is reaching towards the flame like the dark side of the moon to us. So it will be easily 10 °C or maybe 50 °C less. Now it starts to get hard to get any estimations for optimal distance ... so by getting closer with the flame you would ensure that you get also the *hidden* parts of your Changa well vaporized, but while doing this, you will just overtorch the already exposed areas. So with Changa DMT-Destruction is inevitable
With your oil burner you could indeed check at what distance you get to 160 - 200 °C and then use this distance. It will just take longer for heating up than a thermometer and maybe due to heat dissipation across the bigger device it may not reach that same temperature at the same distance so you would have to get closer ...
Maybe just use a tea-light which you can place on the table and then check at what distance you would get to 250 °C with thermometer and then use this distance for oil burner, which may get up to 230 °C at same height?
But before making too much investions into this, better use a more controlled way of heat delivery with a suitable device (E-Nail / E-Mesh for example) and just eat the Harmalas.