I was testing the mirror option if i can draw symetrically with an odd number of pixels (99x99) or in half a pixel, and results with the left side (for example) bring 1 pixel closer to the center than the right side, making it asymetrical, can you fix this?

Mirroring occurs at pixel boundaries and you can't draw/paint half a pixel. The pixel is the smallest element of an image; it's how it works.