Have a search there's loads of info on here, plus brick works site has a good guide. Depending how retarded your timing is you fine tune with the rear idler pulley, (you need to make sure the front auto tensioner is set correctly first as if not your on a losing battle to start with like I was. On the front auto tensioner there should be an arrow on the back plate and another attached to the middle of the bearing, you slacken the middle nut just enough so the back middle bit can be moved, and with a 6mm Allen key in the hole at the end of pointy middle bit you add or reduce the tensions until the arrows align.
Now the front tensioner is set you can adjust the back idler in the same way, buy slackening the middle nut just enough it can be turned, then with a 6mm Allen key in the hole in the bearing rotate it very slightly and it will adjust the timing. (I can't remember which way off the top of my head, but it's not hard to work out, go one way and if that doesn't work go the other).
The back idler has an arm on it, this little arm in the back plate should line up with the join between the top of the block and the cylinder head, this is mid position. If you run out of adjustment than you will need to re set it to the middle, and do the main adjustment you posted to get you close, then start again on the fine adjustment using the rear pulley (once setting the auto tensioner again).
Once you have set the timing as close as you can to 55, (try and get equal scatter around 55) you will need to re-set the auto tensioner as it will most likely be out.. This will then possibly throw the timing out a little bit so again re set it with a tiny tweak on the rear idler.., which my then alter the auto tensioner once the van has been started, so keep tweaking both unti the timing is set and the auto tensioner is set.
Now in theory if there is 2 folks you can tweak the rear idler while the engine is running to find the magic 55, as I was on my own I found out far safer to start the van, check the timing (when hot), note which way it needed to go, turn off the engine, make the adjustment, re start, blip the throttle and then allow to settle down, note any changes needed, and repeat.
Also note that the idle IQ will have a small effect on the timing scatter (or at least it did on mine) so make sure that is set correct already (normally between 3 and 5), and the health of your injectors will make a big change too, 88bhp injectors give up at about 100k, mine on 120k was smoking on idle, a sod to start some times and had big timing scatter.. New injectors and it starts faster than my modern diesels, no smoke and my timing is now only jumping between 49 and 61, not the full graph, so again something to keep in mind.
