Pull your downpipe out of the car and find a small mirror or piece of plate glass. (it won't be flat, but it'll be flat enough for this).
Tape some fine grit wet/dry sand paper on the glass TIGHT and make a few easy figure 8's on the surface. The high spot will light up like a Crimmus tree.
Do NOT get a sanding block or a piece of 2X4 and make it ''look clean and flat''.
Once it's actually flat enough, skim coat some RTV on there, let it skin over and put it back on the turbo. If you bump the flange and 'cut' the RTV, pull it off and start again.
Once all 4 bolts are started, let the two surfaces meet each other without torquing them down. Once they're friends, bottom all four bolts, drink another beer, then torque them down.
That area isn't hard to seal. There's not much pressure there. You just can't grab a ziz wheel, go to town on a machined surface, then expect them to mate correctly.