If you have 0.1 btc in your wallet and trading account before adding 5x(1:5) leverage your position would be $9,000((0.1x18,000=1800x5) not $18,000 assuming you're using your full account balance, so you are using 10x(1:10) leverage if you position actually is $18,000.

BTC position generally will be generated after you apply your preferred leverage and the amount of btc you want to use to create that position, so that doesn't matter how much you have in your balance(wallet) before adding leverage what counts is your position size which in your case that would be 18,000$ (if the above data is accurate and the exchange says you have $18,000 open position), so the result of the calculations would be the same as I said in the previous post.

So if you want to open a position you have to first know how much of your available balance you want to risk and multiply it to the leverage you want to use(5x,10x and such) and that gives you the position size which you can use to calculate your profit/loss on by deducting the trading fees (if the trading fees is 0.1% then it'd be 0.2% for both opening and closing position) and funding(if you're paying funding you deduct the percentage and if you receive you add the percentage) from.