The way to show only part of a texture is to change the UV coordinates of the Mesh the object is being applied to.

UVs work from 0 to 1, where 1 is 100%. So if you want to show 45% of the texture on a quad, you would have to change the UVs of the quad to:

Vector2[] UVs = new Vector2[]
{
    Vector2(0.0f,  0.0f),  // Bottom left
    Vector2(0.0f,  0.45f), // Top left
    Vector2(0.45f, 0.45f), // Top right
    Vector2(0.45f, 0.0f)   // Bottom right
};

This is assuming the quad you're applying it to's first vertex is the bottom left and runs clockwise through to the fourth vertex, which would be the bottom right.