Calculating a percentage

I am writing a spreadsheet for a game I play that will help me decide which buildings to build, it's an easy calculation but problem is is I haven't messed with excel in so long, I can't think of how to do it so any help is appreciated.

Ok here is the problem, I want to

Buildings \ My Aim \ Land \ Current \ Build

Buildings are the name of the buildings, my aim is the % I want, Land is how many I have built already, current is the % it is at now, and last is build, it will tell me how many of each to build.

What I am doing is picking a build strategy and sticking with it and when I attack and new land comes in, I input how much it is, it adds to total land then the calculation tells me which buildings to build.

Example:
Buildings \ My Aim \ Land \ Current \ Build
Homes \ 30% \ 40 \ 10% \ ??

Say I input 400 total acres, that makes it 10%, I need to figure out how to tell me how to build the other 80 Homes to get it to 30%.

Thank you in advance, it is appreciated.