https://shapeshift.io/rate/usdt_btc seems to be invalid at this moment ({"error":"That pair is temporarily unavailable for trades."}) >> But depends after F5 it could appears.

Issue is USDT is a coin (Tether) and not real USD. https://www.coingecko.com/en/price_charts/tether/usd Currently value is close to USD but if you look on 60 days chart, you can see you can't rely on USDT to make maths and get the right usd quote.