We play more multi-player games than two-player. Usually we play with 4 but have done 3 and 5 depending on who turns up. The 4-player is usually 54 inches square (coz of the size of our tables), and sometimes we start within six inches of a corner, and sometimes along the middle 12 inches or so of an edge.
We played a 5-player on that table where we just spread ourselves informally around the edge of the table at regular-looking intervals, nothing too scientific. We've played a 3-player on that table, too, it might look a little bit big for that, but the game went well enough.
Last three player game we made a triangle 54 inches along each side, and started from the the middle of each side. That felt a bit crowded but was a good game and we got all the treasure.
We haven't had an issue with a lack of treasure being captured. Usually all or nearly all the treasure gets got.
If I needed to make a 5-player table that was seen to be fair, I'd make the biggest regular pentagon that could fit on the table (handy for Summoners!) and start from the corners.