Doing a private full day (8 hours) tour for 2 at cost of 590 euro with Overlord. Assuming it’s good, what is an appropriate tip for the guide?
This is completely up to you. There is no set guideline. Honestly for a private tour, I usually don’t tip because the guide’s services were what I contracted for in the beginning. Others may feel differently.
The appropriate tip is exactly zero.
What you could do is treat the guide to lunch since this is a full-day tour.
Tipping in Europe by Rick Steves https://share.google/QYr16KXqlXuXKcBxa