Extract Westmalle Dubbel clone recipe advice

I've looked in vain for a widely-admired extract + specialty grains clone of Westmalle Dubbel, my personal favorite (so far). There are some all-grain clones floating around, but so far the extract clone for WD I like the best is this one:
http://www.beertools.com/html/recipe.php?view=9587

I bought the ingredients for this last week, but I'm planning some changes. First, the hops don't match what's in Brew Like a Monk, which says WD uses Tettnanger, Styrian Goldings, and Saaz. So I bought those instead. The trouble is, I'm not too sure what amounts and timings to use. I also would like to do a late extract addition (15 min) for part of the extract and the sugars, but I don't know what amount of DME to use for the boil, or what effect that will have on hop extraction.

I've also decided to use WLP530 (which is the Westmalle strain) for this brew, instead of WLP570. I'm not too sure why the recipe author used a strain other than 530.

I'd appreciate any advice.

Thanks

Mark Johnson
Silver Spring MD

Sounds like some good changes.

For the hops I would guess a 60 min addition to hit your IBUs would be about all you need, although a small addition 10-20 minutes from the end wouldn't hurt (.5 oz of any of those hops would be fine).