IMHO, it really isn't a matter of getting these paid for as separate line items as it is getting paid for the appropriate amount of materials needed to replace the roof. This BS of "10% waste for ALL gables, 15% waste for all hips is good enough for most" is just that, BS. I put together a document using Xactimate sketches and math PROVING factually this is BS. A simple gable with two slopes, two dormers cannot be built with 10% waste if you assume 5% only for trim scrap. Now, before someone steps up and says "My crew can sure do that", perhaps you're right. If they took great pains to salvage every single piece of scrap, perhaps they could. By and large however, it won't be done.
We have and still use multiple approaches to this. We use the waste calculation spread sheet I developed to show the Adjuster on each claim what the appropriate waste factor should be. That has worked with a reasonable degree of success with some insurance companies, some adjusters. Some adjusters buy understand it, accept it and then make it up with some other line items because their company policies won't allow them to follow our FACTS.
We also create estimates where we put in a lower percentage of waste for the field shingle multiplier (start at 7.5% for low complexity gable, 10% for cut up gable, 12.5% for multi level very cut up gable as examples) and then break out starter and ridge cap separately. We include the statements under the ridge cap and field shingle line items "This item cannot be bundled into waste with the field shingles as it is a different material with a different Xactimate labor rate". Some have bought into this, some don't.
Here's the deal, at least the way we look at it. If they buy in and write the scope of loss accordingly, great. If they don't and we end up going over and supplement, we have convincing evidence to back up our request for more money to cover the additional materials. If we find a certain Adjuster or Insurance company in a given area decides to screw us and not pay, we simply stop work the next job, get the HO involved and not finish until the supplement is approved. Of course that means we need to get paid for an emergency tarp charge and more roof labor hours for the crew. It also means, if explained properly to the HO, the Insurance Company is exposing their dirty little tricks to their Customer Base.
If you're looking for a silver bullet or magic pill that works every time, first try, let me know and I'll also tell you about a great deal I can make you on a bridge in Brooklyn. However, if you're smart, tenacious and a bit tactful, you can get paid what you should get paid 90% + of the time. It sucks that it has to be this way but if you want to participate in the insurance game, either accept lower profits and just bitch and moan about it or take proactive steps to do it the right way. I voted for the latter a long time ago.