Why does this not use exact formulas?
Because the game does not expose them. Skill costs and object stats are confirmed, but how luck sources combine into the HUD figure is not, so the advice sticks to priorities that hold regardless.
Build a Base RNG Base Planner
This one answers 'what should I buy next'. If you want the maths on how many rolls an object takes, use the roll odds calculator instead.
Where are you now
Spend this next
Buy Luck I at 100 coins, then Auto Roll at 250 so rolling continues on its own, then Luck II at 800. Do not spend on base health yet — at 40 placement slots your layout is the limit, not your objects.
FAQ
Calculator FAQ
What is the difference between this and the roll odds calculator?
This one tells you what to spend on. The roll odds calculator does the probability maths on a specific object using the rates the game prints.