New to brewing...serveral questions

Dextrose is a very common addition in an IIPA.  It helps to raise the alcohol without contributing to the body.
“Thick” IIPA becomes hard to drink.  Thinning it out makes it a lot more enjoyable.

I totally agree with all of this. Thick IIPA (or AIPA for that matter) is hard to drink IMO.

IMO, it makes the difference between an IIPA and a BW.

Yep.

Is there a formula for the dextrose addition?  The clone recipe said 5%, but of what, wort volume?

typically you see fermentables quoted in either direct weights or percentages.  When they are in percentages, that would be percent of the overall fermentables in the grist.  Usually, its just the percent by weight of the total grist.  1 lb in a 10 lb grist is 10 percent, etc.

I’d have to look up the number, but I believe it will be 13.5lbs of grain and 4.5% dextrose…thinking about 9oz

I have to agree with this suggestion.  You have picked an incredibly ambitious recipe for your first batch.  I have no doubt that you can pull it off if you are set on this recipe, but your day could end up being quite hectic.  Let us know how it turns out! I’m confident whatever you make will be beer, and it will be enjoyable.