A bonus bet is not cash. If it wins, the book keeps the stake and pays you only the winnings, which is why a $200 bonus bet is worth well under $200 to you. This works out the bet you place on the other side so you walk away with the same money whichever way the game goes.
A $200 bonus bet is not $200. Put in what you have and what you can cover it at.
The book's money. If it wins you keep the winnings, not the stake.
That is what the math says. Real prices move while you are placing, and an exchange takes a cut on the cover leg, so your number lands close to this rather than exactly on it. Anything at or above 70% on a bonus bet is a good one.
Put the promo on something long, like +450, because the payout on the win side is what you are converting. Then cover it on the short side, like -500, at another book or an exchange. One of them lands. You keep the difference.
On a bonus bet, anything at or above 70% conversion is worth doing. Below about 60% it is usually worth waiting for a better line rather than forcing it.
It does not find the cover price. It assumes you already have one. Finding a price that leaves you 70%, across every book you actually have, on games that have not started, is the part that takes software.
Prices move while you are placing the second leg, and exchanges take a cut of the winnings on the cover side. Both are small. Neither is zero, and anyone telling you the result is exact is not being straight with you.