Hard disk space increase

Hi tricky one (for me).

I have a few systems needing to have 20% free hard disk space.
The systems are already in use and I cannot delete files.
So the Final total size of each disk must have 20% free.
Not as simple as getting a 39GB Drive and clearing 20% on it, since there is (for example) already 6GB in use..
I tried to write an excel formula, but I'm not sure this is straightforward. Can some one help.
I have been given figures for: 1. total disk space 2. Free space (GB) 3. Free Space (%) and I need to increment the hard disk size (new total) to have 20% free.
john mastertonAsked:
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Aaron TomoskySD-WAN SimplifiedCommented:
Correct, in this specific case, 125*.8 = 100. However if the 20% is arbitrary, and needs to change, I thought the actual formula would be handy.
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Lasse BodilsenSystem AdministratorCommented:
Something along the lines of.

Total Space - Free Space(GB) = Used Space
then
Used Space + 25% = New Total
and
New Total - Total Space = Needed size increment.

When you add 25% to the Used space, you will get the figure for how big the drive space is needed to have Total size - 20% = Used Space.

in excel:
B1 = Total Space
B2 = Free Space (GB)

then the formula for Needed increment would be: =(B1-B2)*1,25-B1
and for New Total, just remove the -B1 at the end.

EDIT: Added excel info.
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Aaron TomoskySD-WAN SimplifiedCommented:
To get the percent free to be what you want after expanding you can't just multiply by the percent increase as the total size increase changes the percentage.
If you have a 100gb drive 100% full and add 20% (100*1.2) that's 120. But 100/120 = 16.7% free

The trick is to do this:
Current size / (1-percent increase)
To find the percent increase just do:
Desired percent - existing percent
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Lasse BodilsenSystem AdministratorCommented:
@Aeron
That's why i multiplied by 1.25 to add 25%  

eg. 100*1.25 = 125  
and 125 - 20% = 100.
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john mastertonAuthor Commented:
Thanks to everyone!
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Lasse BodilsenSystem AdministratorCommented:
no problem.

even though i think you choose the wrong comment for the "Accepted Solution"
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Aaron TomoskySD-WAN SimplifiedCommented:
Agree, our first two comments are the good ones. The selected answer is just some additional discussion
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