I recommend against using this website as it contains bugs and cannot be relied on. Other miles calculating websites seems also to be problematic. My MO is to calculate miles with gcmap.com and lookup the multiplier on TK website. Manual but mostly accurate (as gcmap.com and TK don’t agree where IST is located).

EDIT: After more testing it, the bugs I found in the tool were not really bugs but my misunderstanding of the tool. I was quick to call judgement. Nevertheless, I prefer my MO due to:
1. It’s easier to enter local info on gcmap.com and see on the map that I haven’t made a mistake.
2. I usually figure out the mileage prior to buying a ticket. Therefore, I not only need the amount of miles for a specific class but see a broader view especially which booking class group give a larger multiplier. For example, having TK website propose a T class flight, I would not only like to know that T class gives 50% but also that the next class in line Q would give me 100%.