How to find minima for non scalar-valued

I want to find minima. My code:

Formula = N[-{{{0.0, 0, 200.0} - 800.0 {0, 0, Cos[Tetha]}}.{Sin[Tetha] Cos[Phi],  Sin[Tetha] Sin[Phi], Cos[Tetha]}}]  DensPlot = DensityPlot[Formula, {Tetha, 0, 180}, {Phi, 0, 360},  ColorFunction -> "SunsetColors", PlotLegends -> Automatic,  FrameLabel -> {Style["θ0 (Degree)", FontSize -> 16],  Style["ϕ0 (Degree) ", FontSize -> 16]}, Mesh -> 10];  DensPlot 

So I plot graph: enter image description here

And now i’m trying to find minima. How we can see it is at 70 degree θ0.

Minimize[{Formula, 50 <= Tetha <= 100, 0 <= Phi <= 360}, {Tetha, Phi}] 

I have next mistake:

Minimize::objfs: The objective function {0. -1. (200. -636620. Cos[Tetha])  Cos[Tetha]} should be scalar-valued. >> 

If i tried to use FindMinimum:

FindMinimum[Formula, {{Tetha, #[[1]]}, {Phi, #[[2]]}}] & /@  {{0, 180}, {0, 360}} 

I have next mistakes:

FindMinimum::nrnum: The function value {636420.} is not a real number at  {Tetha,Phi} = {0.,180.}. >> 

and

FindMinimum::nrnum: The function value {636420.} is not a real number at  {Tetha,Phi} = {0.,360.}. >> 

So my queation How I can find minima?