User Input form into formula

I am putting together a modelling spreadsheet that will show the cost of items we currently buy if they go up by a certain percentage. Since this is sensitive I have made a very basic model of it. I have a couple of questions:

1) How can I take the user's input from the UserForm1 and put it into the formula =(B5+(B5/100)*2) (The red two is percentage of increase that I would like the user to be able to input via the form)
2) How can I get this to apply to all of the items in the list?
3) Is it possible it possible to be able to revert back to the original figures as this is only for modelling purposes?

I see what you're saying but the spreadsheet is so large I cannot add another column. I need to something that will change this one. The formula I'm using is also more complex as it requires different inputs for difference item types, so it's not the same percentage increase across the board.

I don't want to move columns around as a summary is made in another spreadsheet using data from this one and I think it uses hard coded cells rather than using lookups.

I assumed that would be the case. Adding a hidden worksheet isn't an issue.

Okay, I would still like to know how to take user inputs from forms and use them in formulas. If you, or anyone else could advise on that.