Photoshop: Mirror image help please

Was wondering is there any photoshop plugin or other software that i can use on computer to create mirror image?
I am trying to do some mirror image with my art works but using photoshop ‘flip horizontal’ or ‘flip vertical’ it just not very user friendly to me since im trying to adjust the picture and flip it at the exact pixel.

I do this picture below using an app on phone. Which is very user friendly and easy to adjust. But unfortunately the resolution is not good enough for prints for wall art.

So i rather do it on my computer. But flipping it in photoshop, i need to estimate the central line and create a new canvas and flip it but it doesnt always come out great so i have to redo and slice the picture and flip it again. Sorry i dont even know how to explain this. Anyways, is there some new method to do this or did they added the mirror image function in the photoshop that maybe i just dont know about? Pls kindly let me know:)

The app im using is photoshop express on iphone. And is really easy to do it there. But the resolution is too low. I couldnt find this function in the computer version;(

there is another app called skrwt.

watch this video. start at 13:00. 
https://www.youtube.com/watch?v=LEEm7Bz10GY
when you flip it, you still need to adjust it to a place when the centre line doesn't look awkward. see how he move the picture up and down and the other half part just go with it. but in photoshop, if you move one layer, the other layer will block it, they don't move simultaneously but going opposite direction.

Sorry my english is bad. I hope you understand what im asking T___T