scrollbar forumula needs update

so i have some scrollbar(track and button) and some content.
the formula i am using to calculate to calculate y of content is the following:
//contentStarting.y and scrollbarButtonStarting.y are positions of sprites at the moment they are created
content.y = -((contentStarting.y -scrollbarButtonStarting.y) +scrollbarButton.y) * ((content.height- content.mask.height) / (scrollbarTrack.height - scrollbarButton.height));

this forumla works almost perfectly for me.
the only problem i have is that i need to add extra space after content, so basically i want forumla to act as if
content.height = content.height + extraSpace , but unfortunatelly setting the height scalesY, which is what i don't want.
only viabble sollution that i could came up with is to add some sprite at the end of content
with alpha= 0 and height = extraSpace
but that doesn't seem like good sollution to me.
So can som1 of you people good at "simple math" please update this forumula so it can work with extraSpace?