Converting Expense Data Overages to Correct Users - Looking for an automation or simplify

Hello Knuckle. Welcome to the forum.

It is difficult to say without adequate data.
And
Without a date column

Even if someone else used 25GB earlier in the month,

I don't see how this could be calculated.

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Hello Knuckle and Welcome to Excel Forum.
Here is my interpretation of what you have described. (Note that I suggest keep the process simple and not combining into one hard to understand/explain formula)
Column A is a list of 20 names (120/6=20)
Column B shows GB usage
Cell B22 shows the total GB's using: =SUM(B2:B21)
Column C identifies the users that exceeded their 6 GB using: =IF(B2>6,B2,0)
Cell C22 shows the GB's used by 'overage users' using: =SUM(C2:C21)
Column D shows the percentage using: =C2/C$22
Cell D22 shows the total percentage (as a check) using: =SUM(D2:D21)
Column E shows the amount each 'overage user' will be charged using: =D2*B$28
Cell B25 gives the GB's allowed (120)
Cell B26 gives the overage using: =B22-B25
Cell B27 gives the amount per overage GB (I just put an arbitrary amount of $10)
Cell B28 gives the total amount owed for overage using: =B26*B27
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@FlameRetired, I was working on this when you responded and did not see what you said until just before submitting.

@ JeteMc. No problem. Looks like that took awhile to organize, type and post.

Haven't looked at your upload yet.