Props on giving a very detailed explanation of what you're doing! With Capella and Flavor West, I'd be hard pressed to think of any of their flavorings that would work for me at just 3%.
There's a whole database here for flavorings, and it's AWESOME.
When looking up a flavor, I always click the recipes column to sort by the number of recipes it's been used in so that I avoid using a duplicate entry with little data. EXAMPLE
Go ahead and click on the top result for Sweet Strawberry (Cap) in that example, I want you to notice two things.
Percentage in recipes
Here it has pulled info from every recipe someone has posted on the site that uses this flavoring. 5.4% is the current average, so that's the average used amount in a multi-flavor mix (that counts single flavor recipes too, but don't worry that doesn't throw the average off that much because...
Single flavor recommendations
Here is where it lists an average from all the people who have entered a single flavor recommendation when entering this flavor in their flavor stash. A lot of people skip this, so there's a lot less data, but in this case 58 people have suggested anywhere from 1% to 20%, but the average is 10.3%. So if you wanted to just do Cap Sweet Strawberry, 10% might be a good starting point for your test.
The other thing you can do, and I strongly recommend this, is enter the flavors you have into your flavor stash. This gives you a place to take notes on you different flavors and enables the use of the What Can I Make feature of the site.
One last thing, having your percentages too low shouldn't result in the harshness you describe, so there may be an issue with your nic base, PG, or VG. Try vaping some of your VG by itself, try a 0 nic PG/VG mix, and then try mixing some of your nic without using any flavoring. If any of these vape horribly, the problem isn't your flavors!
Hopefully this has not been too big a wall of text, so I will stop now before it's much too late.