Calculating correct exposure with Flash ?

I understand that FF bodies have some advantage when shooting low lights. But for people who dont have FF bodies to work with what is option ? I've got Sigma 30mm F1.4 & Canon 85mm F1.8 on Canon 60D and I personally start hating noise even in indoor personal shots above ISO 800. So how are you guys getting away with 3200 & 6400 ISO ? Please share some of your images and processing tips to help us newbies

Also I use 580EX II and try to follow 1/F rule for shutter which gives me in my case 1/30 (generally I tend not to go anything below 1/60 as I shoot only handheld) or 1/85.

Now with regards to getting correct exposure - how do you calculate that if you are using external Flash ? Here is my confusion. When I use flash (and I shoot only in RAW & Manual) I set my shutter (try above 1/60) and aperture (most of time stop it down 1 or 2 stops) with Evaluative metering mode. Many times my meter inside viewfinder shows that image is underexposed but in this circumstances I rely on my 580EX II to compensate for exposure light.

So technically speaking I'm letting my Flash guide my exposure under low light conditions.
Is there anything I can improve here ? What do you guys think ?

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Thanks
M