If you're using a phone you can always try printing it out and putting it up so you can measure. Or if your screen you use is big enough then that works as well. A good thing about photos is that its easy to compare your proportions by overlaying the image with your drawing. Otherwise if that's still not helping ask someone to pose for you (I know this can be a bit easier said then done but it doesn't have to even be nude). I hope this helped and that i've addressed accurately what you are asking for advice on