I'm making a really low poly character, he is literally made out of cubes with various size vectors. I want to be able to draw on him using a 64x64px image which is basically pixel art and I want the pixels to fit perfectly with the UV. I've tried everything, even making the image 100x100px and then making the size of the object rounded to one decimal so from 0.1 to 1. All I need is a formula that I can use to get the closest value for each axis (X, Y, Z) to which the pixels will be aligned in a 64x64px image where each limb is a separate cube with it's own texture and UV Map. For example in the X axis of my arm the size I gave it is 0.3 and for it to perfectly align the UV with the texture it must 0.325(example). Or if there is a simpler way do this please tell me. Thanks in advance :)
If it's sufficient to align the UV islands to the pixel grid of the image, then you can snap selected vertices to the pixel grid with Shift+S > Snap Selected to Pixels.
Additionally, there's the option to snap the vertices with every transformation by activating UVs > Snap to Pixels in the menu of the UV/Image Edtior.
If the proportions within your texture are important, you should make sure to align and scale all islands so that they approximately lie on the grid before you snap them to it. Otherwise your islands will be distorted: