How do Christian proponents of the Fine Tuning argument for God justify their probability estimates?

For reference, I am not a Christian but I am interested in hearing theistic perspectives on this. The fine tuning argument essentially goes that the probability of constants in physics resulting in life is very low given all possible constants. On the other hand, the probability of these constants is high given that a designer existed and wanted to produce life. But probabilities depend on sample spaces. Who gets to define sample spaces? When we calculate the probability of life producing constants, we implicitly think that the sample space is all possible real numbers. Why? Secondly, what is the sample space when calculating the designer’s intentions and more importantly the sample space of events when calculating the designer’s existence? Why isn’t there a global sample space that includes all kinds of different universes with different constants but also god? Different answers to these give wildly different probabilities. The probability of a dice landing on 2 is 1/6. The probability of a unicorn magically making the dice land on 2 if it existed and wanted it to be this way is 1. But what is the probability of this unicorn existing and what would be the corresponding sample space? It seems as if answers to these questions are arbitrary. If the sample spaces cannot be justified, how is there any justification for any of the probabilities used in the fine tuning argument in the first place?