I agree with your approach. For an APA with lots of local, leaf hops where I don't know the IBUs, I always liked to get my "guaranteed" IBUs from a single ounce of a store bought bittering hop, aiming for the low end of the style. Say 25-30 IBUs from that bittering hop. Then throw a boatload of the leaf hops in at flameout (4-6 ounces). Your real IBUs will come out somewhat higher, giving you about the right gravity: bitter ratio, with a ton of awesome hop flavor. Using the unmeasured hops for all flameout additions or dryhopping lowers the range of potential IBU outcomes.
Personal preference, but I like recipes with light malt bills like 90% base, some light Munich and wheat. Usually SG of 1.05-1.056. Lets the hops take center stage.