Im sorry but we can't get you out of the habbit of wanting to make everything perfect to the inch, depending on how you work will depend on what you can do to try and get out of that habbit, for example, I can surgest that you try to use less referances and rely more on your artistic knowlage of the final result not the imidiate problem of size.

I say this because I belive that the more you use referances the worse it will be for you to get out of the habbit of not worring about scale (its easy to guess a size without an image in the background that has a specific length.)

I'm not saying dont use referance images im just saying keep them as visual insight not as size calcuations. and that might help you get out of the habbit.

(Another thing to note is that images always lie, if you have a front and side view of a human head for example they wont line up so its pysicaly impossible to follow both images as a referance.)