Indeed I check the spread between buy and sell at money exchange booths. In Thailand it can be under 1%, in airports in Europe I saw spreads of 30%.
In your particular case you might get a better rate than I do, but your method is not generally applicable as it works only for your combination of home currency and home bank.
Currently on Superrich, Euro 100-500 notes: buy 37.6; sell 37.65; that's a spread of 0.13%;
xe.com 37.71 THB for 1 Euro; that means on superrich I lose 0.3% versus xe.com.