How is % completion calculated?

I'll need more details on what you are seeing, that formula should be correct.

hi - I have a project with 26 actual and 34.5 remaining. showing 49% but wouldn't the calc be:  26/(34.5 + 26 = 43%?

Do you have empty milestones in the project?  This will change the weight of the sub-items.  There is also a feature called progress impacts that allows you to make some children work items have more weights then others when rolling up to the parent.