DS90CF366: Power Dissipation/ Junction Temp Math Double Check

Part Number: DS90CF366

I wanted to verify two quick questions with the TI team/ greater community related to junction and max temp on the DS90CF366 IC. My company is running into a potential problem where we might be overheating this IC - I had two questions below since I haven't touched thermal items related to a board in a few years:

1. I thought that the junction-to-board and junction-to-case temps summed up to the junction-to-ambient temp - but in the case of DS90CF366 case they don't - 22.1+34.8=56.9 whereas the junction-to-ambient is 67.8. What am I missing?
   1. This is important because my company may be running this IC at an ambient of 45°C in some cases. Assuming the power dissipation of 1.61W. The total temp at junction at 45C could exceed the limit of 150C, which is obviously a concern. However, i want to verify that my math is correct. Absent a thermal analysis in Solidworks or Ansys, I have to assume the Power Dissipation at ambient of 1.61W and the junction resistances provided is accurate at the hotter ambient temp too.

Thanks,

Adam