Wilton's is short, I've heard, and I've used Earlene's but she doesn't have as many size options (looking for 3 tier square for 125 people).

Any other reputable cake serving chart out there??

Squares are fairly easy, using a 1"x2" 4inch high as a serving.

The accuracy of the cutting chart also depends on the skill of the server.

I usually go with a number between Wilton and Earlene.