Ann: June 2021 Investment Update, page-53

Correction to this bit.....

With dividend
.....
$16499.5 normal total tax on income ( excl medicare )
$16499.5 + $1602 - $1080 = 17021.5 ( including medicare and LMITO)
17021.5 / $80100 = 21.25% effective tax ..... eg the tax are actually liable for on taxable income.

Balancing the tax account.
$17021.5 - $30 = $16991.5. <- Tax payable minus all credits/tax withheld ( like franking credits )

Difference in tax payable on taxable income? $34.5
Difference in assessment amount $4.5
At face value you will think that means you paid 34.5% tax on the $100... but this is misleading.
Effective tax rate is the amount you actually pay across your entire income.
That $100 is not special... is it the first $100 of income on which no tax is paid? or the last $100 on which 34.5% is paid?
The answer is neither.... you are taxed on the total taxable amount using the progressive tax table calculations to get to the effective tax rate.
The calcuations make it seem like you are taxed 34.5% but on average you pay only 21.25% on any given $100....including your grossed up $100 dividend.

Applied to franked vs unfranked.....

Cashflow for franked $100
$70 <- paid to you
$30 <- paid to ATO
$100 <- Assessable income amount
$100 * 0.2125 = $21.25 <- effective tax rate
. $30 - $21.25 = $8.75 <- refund
$70 + $8.75 = $78.75 <- amount in your pocket.

Cashflow for unfranked $100
$100 <- paid to you.
$0 <- paid to ATO
$100 <- assessable income amount
. $100 * 0.2125 = $21.25 <- effective tax rate
. $21.25 <- payable TO the ATO... no refund.
. $100 - $21.25 = $78.75 <- amount in your pocket....

Hope that all makes sense now, best of luck.