A proportional target needs the original width and height plus exactly one known target side. Supplying a target width calculates height; supplying a target height calculates width. Two target values would no longer leave a missing dimension to solve.
Calculate height from a target width
Use target height = target width × source height ÷ source width. For a 16:9 source and target width 1000, the missing height is 1000 × 9 ÷ 16 = 562.5. Keep 562.5 as the unrounded result before applying a whole-number rule.
Calculate width from a target height
Use target width = target height × source width ÷ source height. For a 16:9 source and target height 720, the width is 720 × 16 ÷ 9 = 1280. Because the result is already whole, every rounding rule returns 1280.
Keep one source of truth
Choose the target side fixed by your layout, then let the ratio determine the other. Do not enter two independent target sides and assume they preserve the source ratio. Copy the calculated pair only after checking the unrounded value and rounding rule.