Refactored `_percent_of_viewport_height` to remove redundant calculations.
Refactored `_percent_of_viewport_height` to remove redundant calculations.

alanwsmith/document.write-image-loader,alanwsmith/document.write-image-loader,alanwsmith/doc-write-image-loader,alanwsmith/doc-write-image-loader,alanwsmith/doc-write-image-loader