There is an interesting post by zoetmb here about the math 90% surface vs. 95% vewfinder coverage:

"If there is 95% coverage in each direction, he would be correct from the perspective of area. Here’s how to calculate it: The Nikon FX frame is 36mm x 23.9mm resulting in a total area of 860.4 sq mm 95% of 36mm is 34.2mm. 95% of 23.9mm is 22.705mm. 34.2mm x 22.705mm = 776.511 sq mm