Can't get my head around this asset allocation formula

I have a small spreadsheet to figure out asset allocation for a retirement account. There are a total of ten index funds and I have assigned a percentage of the total value of all funds that each fund should be. For example:

Total is $1,000
Fund 1 = 10% (or $100)
Fund 2 = 5% (or $50)
Fund 3 = 7.5% (or $75)
etc.

Once a year, I will rebalance all funds to match the percentages by selling funds that hold too much and redistributing to the funds that have too little. This is very simple to calculate; just total everything up and multiply by the target percentage. However, on a monthly basis I want to add money to the funds and keep the balances as close as possible to the target percentages but (and this is where I'm having trouble) without selling any of the funds that are over the target percentages. For example:

Current total is $1,000
I'm going to add $500
Fund 1 goal is 10%; current is actually 5% (it's 5% under)
Fund 2 goal is 5%; current is actually 4% (it's 1% under)
Fund 3 goal is 7.5%; current is actually 9% (it's 1.5% over)

In this case, I would want a formula that tells me how to divvy up the $500 contribution so that after the contribution were made, the percentage Fund 1 and Fund 2 were under are as equal as possible (e.g. both 2.3% under). In most cases, I would need to ignore funds that were over their target percentage (since I'm not selling), but if a contribution were big enough and that fund was just a little over the target, it might need a portion of the contribution as well.

Any ideas on how to solve this? And I apologize if my explanation is unclear.

Thanks very much for your reply, xladept. Not sure I follow your suggestion, though. In the example, I only have $500 to apportion, since using the whole $1,500 would likely result in needing to reduce the holdings of some funds (i.e. sell). That's something I only want to do once a year. Am I misunderstanding?

Thank you!
Michael

xladept - I guess I was overcomplicating things. Thanks for your help with this!

Michael