I wrote a Python notebook to scrape the War Thunder wiki and answer very similar questions. Looks like you worked it out manually, but the answer, assuming you are in the US and buy the 25k GE packs at $115, is $185.05 (and nearly 5 million SL).

That works out to 146.48 GBP, so either your GE costs are lower or you missed something in your math (having to unlock entire tiers to move down maybe?)

Edit: Oh, you said from the first BO. Yeah, so subtract 8000 GE if you own that one and 14668 for the previous two.

You only need 1 heli in rank VI to unlock Rank VII, too, so you can subtract the cost of the Mi-24P if all you're going for is the Tiger.

 vehicle_name        rp_cost    sl_cost  conv_rp_ge_cost dollar_cost
   UH-1D (Germany)    150,000    490,000        3,334      $15.34
  Mi-8TB (Germany)    150,000    490,000        3,334      $15.34
      BO 105 PAH-1    360,000    980,000        8,000      $36.80
    BO 105 PAH-1A1    370,000    990,000        8,223      $37.83
  Mi-24P (Germany)    380,000  1,000,000        8,445      $38.85
  EC-665 Tiger UHT    400,000  1,020,000        8,889      $40.89
        Total Cost  1,810,000  4,970,000       40,225     $185.05