You really don't need to tamp them more than it takes to get all the powder in the capsules - you want them honestly full, that's all it's for.
Flush with the topn edge of the long half of the capsule, yes: it should weigh out at roughly 575mg to 650mg, but because of density, humidity, the character of the compound (is it grainy, powdery, crystalline, wet, sticky?), the weight varies, and might vary more than that. I believe what you;re wanting to cap is dry & has a fine granular quality, it should be a dream to work with...as these things go.