Indexing 6 sprocket rear derailleur, having trouble reaching largest sprocket I'm trying to index my rear gears after replacing my 6 sprocket freewheel. I've set the limit screws fine, but I didn't have enough play in the barrel adjusters to make much adjustment, so I undid the clamp to release the cable, tightened both adjusters up then let them out a couple of turns, then reattached the cable finger tight. 

I'm finding, though, that I can get the smallest cogs to change okay, but by the time I reach the lowest gear, it won't move to the largest cog, just this awful clacking of gears, even though by eye the jockey wheel is in line with it and the b tension gives plenty of room, about 1cm. 

I thought I was meant to tighten the cable with the adjuster to amend this, but nothing makes a difference, I feel like I need to loosen or tighten the cable overall with the clamp bolt, but I don't know which way to go, am I doing something drastically wrong here?
Returned to this this morning, and have found that the high limit screw could be let out a bit more, and now that is done the chain reaches the large sprocket just fine! Apologies for being dense!

I am now finding that I can index it fine for going up from the low sprocket, or coming down from the large sprocket, but not both, but I'll work on that....