lower FPS -> higher Delta

x = velocity * delta;

higher FPS -> lower Delta

x = velocity * delta;

x = x ??

That is correct, unless the velocity changes due to acceleration within the time period indicated by delta. The mathematically 'correct' way to handle such situations is quite complex (at least for people like me who are not overly mathematically inclined) involving integration and other stuff to avoid different deltas having different effect. Even then, collision detection brings additional difficulties.

By far the simplest way to do this is simply

**fix your timestep**. If delta is a constant, small value, you wont have to bother with all the mathematical complexities. Take a look at the game loop ra4king posted earlier

here, it's simple and it works. I use a loop based on this one, with some minor adjustments.