On Cryptowatch (or equivalent): see percentage change to custom reference point Given that I bought a certain amount of BTC at a given point in time. Is there a way on cryptowat.ch or an equivalent platform, to mark the price and point of time at which I bought the BTCs and get information about how the current price is to the reference price in percent (preferably live)?
Not the way you are putting it but i update my portfolio on 
https://www.worldcoinindex.com/portfolio

This allows me to see in euro/usd and others.
For example you bought 1000 usd of any combination of currencies.
This profile wil tell you what it's worth approx (different markets have slight changes between them).