Proportions are sadly pretty mathmatical in nature, so keep in mind that all of what i discuss should be done only in refining and not ideating stage of your artwork. I would suggest you that after you have a loose sketch of what you are drawing , use guidlines over your subject to correct proportions. To do so, draw a box of suitable dimensions over the subject. And subdivide it into its halfs, now from this guidline, you can now compare the respective " Proportions " of the parts of your subjec,t like the size of eyes with respect to the nose , and (specially for stylisation as in anime ) then cohere your subjects proportions to what you would like to have ( whether from imagination or from reference, this is a choice you gets to make). For your concern over proportions in perspective, you could apply the same principle in perspective, as long as you can subdivide a box's planes into subdivisions, you can do the same in perspective by extending the subdivision, as if a small slice of the box, in perspective towards the other Vanishing point. So the process of dividing the box into subdivision is as below : 1. Draw the box with the same dimensions ( like the heights proportions of the box to the breadth of the same box ) as the dubject that you wish. 2. Draw a cross over the desired plane of the box by connecting its diagonal ( as illustrated) 3. Now by using the middle points of this cross ( the intersection points of the cross ) divide the square plane into its half segments. Repeat with the new half planes to subdivide further Now for 3d box instead of a 2d rectangle : 4. Extend this half division in 3d space by using the other vanishing point to make a " slice " at the middle of the box ( can be applied both vertically for vertical proportion or horizontally as illustrated ) . 5. Now cohere the respective proportions of the parts of the subject respective to the other parts, like the eyes in the middle point, about one-forth , the nose about one-third and so on. If this sound tedious, well that because proportions in perspective are. That is the reson we wants this to be after ideation and in refining .But as you does this as a part of your process, this slowly becomes more and more intuitive to just imagine instead of drawing the whole guidlines