Start with the requirement, not a random quality slider
The useful target is usually written on the form you are trying to submit. Record the minimum, maximum, accepted format, and dimension rules before editing the image.
Browser tool or desktop editor?
A browser tool is fastest for a precise KB target. A desktop editor gives more visual control when color, cropping, retouching, or print preparation matters as much as file size.
Understand the four checks behind a successful upload
File size, image format, pixel dimensions, and aspect ratio are separate requirements. A result can be exactly 50KB and still fail because the form expects JPG, a portrait crop, or a particular width and height. The guides explain how to read all four conditions before changing a source file.
They also distinguish minimum-size rules from maximum-size rules. A minimum can often be met without changing visible pixels, while a strict maximum may require a quality or dimension trade-off. Start with the general guide for the decision process, then use the Photoshop article when you need manual editing control.