Keep symmetry no matter whats the angle!

I have this very easy problem to you guys to solve I haven’t found anything via google or in this site and I haven’t found any way yet to solve it my self I bet there is something about this problem, but I don’t even know how to search it right so…

I use letter “v” in my example it will explain the problem.

So let’s say I want left side of the letter “v” to be more down than right side like this (pic A)
Now the problem is that how I can move the left side or anything that is in “angle” in blender in the way that it keeps the perfect symmetry all time!
As you see the lines in the left side of the letter aren’t in perfect symmetry to each other. Lines are opening more downwards than they are on the up. So the problem is how to move anything that is in some sort of angle so the object keeps the perfect symmetry ?
I moved the top 2 vertex in the Z axes to get that result, by hand it’s even more difficult. Also scaling doesn’t help so I don’t know what to do. To me this so fundamental dilemma that there has to be some sort of option to do it right and keep symmetry all time ??? When objects don’t have lines that are in some sort of angle then the symmetry is easy to keep just by using purely axes, but it doesn’t work on angled objects

Attachments

press k, then move the mouse to the dip point of the V, and then press c, and then drag down, and click

you would have drawn a line there

I get knife tool workin fine it gives the possibility to adjust the second cut point when pressing C. Edge slide doesn’t seem to keep the vertecies in vertical position instead it when I drag down moves left vertex more down than the right side vertex ? I don’t know does the even mode keep the vertical position straight I don’t understand how to get into that mode ?

Don’t forget that you can also select an edge and use it to define a custom transform. Then while that transform is selected, you can do a move/scale/rotation operation in relation to the axis by pressing the appropriate axis twice or using the transform handles.