Those of you out there running a SC12 I am looking for information on what pulley sizes you run and how much boost you get on the particular pulley.

don't forget that boost depends on engine capacity - mine is driven very close to 1:1 using the standard SC12 pulley - I get 9psi at about 6500rpm

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Which engine do you run that on? I am planning on running one on a 1330 in a blowthrough setup and want to get around 8 PSI at 3500 rpm.

Blowthrough setup? That's going to be an interesting plumbing exercise.

Are you wanting 8psi max, reached at 3500rpm? Like a turbo achieves with the use of a wastegate?

I don't know what you've got planned, but it sounds to me like you want a turbo.

Yeah I thought about turbo but decided to try something different, If it doesn't work it won't have cost me much and I will have learnt some new skills.

You could simply mount the SC at the front of the engine, run through a inter cooler or just a pipe to the sealed carby but between the SC and the carby fit an adjustable pressure relief valve that you can tune to what ever psi you want.
Seeing as it's a blow through the dumped air will not be "fueled" so it'll be safe to dump into the air.

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That is what I am working on but I also want it to be able to run with or without the charger engaged. I.e have it switchable between all the time, some of the time, and not on at all.

mine is 1330 -

you'll need an intercooler if you want 8psi at 3500rpm, because you'll end up with sh!tloads of boost at 6000, more than the blower is meant to be able to handle

look at an SC14 instead, but will still need cooling

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The engine will still run if the SC is not being driven by the belt ie sucking through the SC but it'll be sluggish and cause poor economy.
What you will need to do is set up another inlet that "Y's" into the charged piping (between the SC and the carby) and this will need some sort of electric switched gate/valve thingy that opens when the SC clutch is disengaged. Or maybe a vacuum operated gate, I don't actually know what would be the proper/best way to do this.

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same as the way Toyota do it - has a butterfly valve in the intake bypassing the blower when it's turned off

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Yes that is what I have planned and the valve will also bleed off extra boost, but for the moment I am trying to work out pulley sizes which doesn't seem to be working for me, through my calculations I always seem to end up with stupid amounts of boost even with a 1:1 ratio.