If you want any chance to success you need to do everything by your self.
Think about it multyple times before posting and try figure it on your own before asking for help.
If that fails we are here.
What you want to do is probably following
Make a sf::RectangleShape and set its sf::RectangleShape::setTexture(to texture of loadingbar)
then each loop sf::RectangeShape::setTextureRect(sf::IntRect(X,Y, Calculated progress, height))
Draw it.
I may not understand your question, its probably lack of information.
You require math to calculate how much of bar you need to draw or?
It above helps, am glad i could be of help, case else i tried.