If it makes sense, a small hyperlink to preev.com for the Crypto Calculator or somewhere on your website will be not bad. No adverse effect for your website, I believe so.
I don't think it would make a lot of sense. NakitCoins is in the business of buying and selling cryptocurrencies, it is quite appropriate that they have their own calculator based on their exchange rate.
Take, for example, changelly or some other crypto swap platform. You don't see references to other services on their website, do you?