Straight knife cut in perspective view? Here are the methods I use to make a straight cut 


switch the view from perspective to orthographic 
just make a cut then straighten it with s+x+0 
shift+7 to align face to view then make a cut 
use bisect tool then change the angle
vertex connect pressing J key and then straighten it 


There are many other methods too but nothing is as fast as knife cut.

Is there any workaround to make straight cut in perspective view ?

I know we can snap knife tool to vertex &amp; midpoint but it doesn't help in many cases. 

Thank-you and sorry for my bad English.
press "c" on the keyboard, then you can make 90 and 45 degree cuts, combine width ortographic view