Binance Futures API. How to calculate market order quantity?

Binance Spot API has a parameter quoteOrderQty which will allow you to say how much you want to spend for an order, like you want to buy $50 worth, you just need to do quoteOrderQty=5

However, Futures API doesn't have this calculation and you need to find how much is the Quantity yourself - docs

Does anyone know how can I calculate the quantity of the order?
I am trying to do it as:

js:

const orderSize = (((percentageOfFunds / 100) * availableFunds) / price) * leverage;

However, I am getting { code: -2019, msg: 'Margin is insufficient.' }

I am pretty sure something is bad in that calculation, probably missing some fee calculation in there because when I do the same request in the binance UI and watch the network tab, I get the quantity being 25.08 and mine is 25.255 at the same time.

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Thank you!