I'm with Likes2Vape on the IR gun and a good starting point.
After a bit of experimentation, I am running my D-nail SiC halo at 640 on an RDK-300 with an D-nail flat coil. Now, I tighten my coils for better heat transfer so the difference between controller set point and actual dab temp can vary between mine and yours for this reason, if you have a fan blowing on it, ambient temp to an extent, yada, yada, yada.
Below are measurements I took using a K-type thermocouple thermometer. Hope this is useful to you but please don't necessarily expect your rig to be exactly the same...mostly its coil fit, IMO. Look to the bottom section for the most relevant data.
Also, I find it helps to preheat the carb cap on the SiC before dabbing...just a bit, you can see temp on the controller fall for a bit then start to climb again. Once it starts to climb again, I'm ready to dab.