Choosing to average down

Your average for a stock is the total amount you've invested in variously priced parcels of the stock, divided by the number of shares you hold.
Say you have 84,000 shares in PLS and Westpac or Comsec have tracked your average to $3.75 per share. You have $315,000 invested.
Recently the share price dropped from $3.75 to $3.50.
You need to decide whether to double down by purchasing $10,000 with of shares at the lower price of $3.50.
How is this going to change your Average?
At share price $3.50 you'll buy 2875 shares, meaning your investment will increase to $325,000.
Divide that by the number of shares you now hold (86,875), and your new average will be $3.74.
You will have reduced your average share price by .. one cent.
Simple calculation for the experts here, but I've always just bought the drop and waited for the screen to tell me the new average.
Given you'll add the 10k worth of shares in the hope the share price will double and you'll receive dividends, doesn't it help to know in advance how little a substantial investment impacts your average share price?