The moment before your first spin
Choosing a bet size is your decision, but the bonus round is not. How often the Ghost Respin comes is not known in advance, and the result of one session is random whatever the RTP. Two players at the same bet can finish far apart after the same number of spins.
Confirm the RTP version before you play. Which version applies is shown only in the game rules, so read them rather than guessing from the reels or the lobby tile. Read the rules once before you stake money, then compare them with the RTP figures on this page.
Setting a target means knowing the ceiling first. The maximum win of Great Ghosts! is x40,000 the bet, and that ceiling is reached very rarely, so it should not shape how you play. The money value of that maximum appears in the table.