Calculating current draw with cascaded regulators (78xx)

[thelonious]

My pedalboard is one of those SKB hard case dealeos with a built-in 9V PSU that works just fine, but each 9V output is limited to 100mA. For analog pedals that works just fine, but I'd like to add a higher current outputs at 9V and 12V for digital pedals. A while ago, I added an 18V output using a 24V wall wart and a 7818. The wall wart is pretty beefy, and the 7818 has a big heat sink (~3" x 2" x 0.5"). I'd like to add a 7812 and a 7809 to this to provide 12V 300mA for a Nova Reverb and 9V 300mA for a Timeline. I've read a bunch of stuff at other sites and also used the search function here, and there seem to be differing opinions about whether it's better to cascade the regulators or run them all off of the 24V source. It seems like cascading might be better since the heat would be distributed across multiple regulators, but that draws a lot of current from both the 7818 (650mA) and the 7812 (600mA). The 7812 will also be heat sinked, but with a smaller sink than the 7818. I looked up the thermal junction numbers in the datasheets and plugged them into a calculator online. It looks like they will all be able to handle what I'm throwing at them, at least on paper.

My questions are:
1) To cascade or not to cascade?
2) If I cascade, can I add the current draw as I go up the chain like I've been doing (300mA+300mA+50mA) and expect that only 650mA will be drawn from the 7818, or will there be more current actually drawn because of inefficiency, etc.?

[PRR]

Each 78xx also pulls a few mA to keep its little brain alive.

So maybe 660 instead of 650.

It's in the datasheet.

[thelonious]

Thanks! Just making sure I wasn't missing anything... sometimes when I read datasheets I say, "Oh, obviously ____"... and then I burn something up when I find out I didn't understand it in application.