I try to calculate my hours then multiply it 1.5. But it's never easy. Clients want to change a lot on the way. Everytime I have to explain them and revise my

I try to calculate my hours then multiply it 1.5. But it's never easy. Clients want to change a lot on the way. Everytime I have to explain them and revise my offer. I try to put milestones to keep track.