I'm sorry for the confusion.

I'll try and make it more clear.
This is a
flat rate (not hourly) pay scale. Some bands like flat rate, since how can they verify your hourly work unless they are there to count hours?

Yes that can be one reason they want to stick around!
So to alleviate that, you make it a flay rate, no matter how many hours it takes.
The scale is to help the band make the right choices about the changes and to prevent micro tweaks. You know, "turn the kick up a teeny bit", then the next mix "Turn it up a teeny bit more" then the next mix, "too much kick". This way you get payed for the tweaks but at a flat rate.
(low prices for example only)
1st mix = $10 =
$10
2nd mix = $10 + $0 =
$10
3rd mix = $10 + $3 =
$13
4th mix = $10 + $3 +$3 =
$16
5th mix = $10 + $3 + $3 + $3 =
$19
etc....
Does it make more sense now?
You can also charge by the hour. In that case it is a constant cost per hour, no graduation of price. But this way is slower, and more costly. However the band CAN sit in on the mixing sessions.
Anyways, some great posts in here. This is one of those important issues that needs to be talked about.