Attaching a Ledger to concrete slab?

Hi all,
What is the best way to bolt a timber ledger to the side of a house slab?
My main concern is when you drill into concrete the drill bit will never go exactly where you want it, so if its a little high it will put the levels out. I was going to drill the holes then measure how off level they are before drilling the timber?

If you're worried about the bit moving when starting the hole, turn the hammer mode off to start with to mark the centre on the concrete then turn it back on

I'd drill all the holes in the timber first, then drill the central hole in the concrete and attach the timber with one bolt. Then level it up and drill the rest of the masonry holes through the timber. If your bit isn't long enough, then pivot the timber (out of the way) around the bolt, or undo the nut and remove the timber to get full depth holes in the concrete.