Jhyarelle wrote:Does this mean that the target's toughness 8 goes against shuriken DC 18 (Base DC 16 +2 for 2 deg success)

And toughness 8 vs shuriken DC 21 if attack roll does 3 or more degrees over target's Dodge?

Yes, but just a few clarifications on your example:

Jhyarelle wrote:My Range Attack is +7

I rolled a 13 on d20

Total Attack roll is 20

Target's Dodge 10

Is that the target's Dodge rank or the DC to hit them? A character with Dodge 10 (a reliable average for a PL10 hero) is DC20 to hit (Dodge rank + 10) vs the attackers Attack bonus + d20 (7 bonus + 13 rolled in this case). However if it's DC10, that means your target has a Dodge of 0, which means they might currently have the Defenseless condition (possibly bound) or be completely unskilled in dodging or an inanimate object.

Jhyarelle wrote:I score 2 degrees of success on the target

This actually isn't quite right for either interpretation:

If target is DC10 (let's say an inanimate practice dummy) then a total Attack roll of 20 would be 3 degrees. It's a little confusing, but the essential rule is that the die roller always wins ties, so a total roll of 10-14 vs DC10 would be 1 degree of success, 15-19 would be 2 degrees and 20+ would be 3 or more. Each degree is 5 steps, but it's easy to miss when you start counting from a zero.

For a DC20 target (say a sparring partner on your team) a total Attack roll of 20 would be a tie, and therefore 1 degree of success.

Versus the dummy with Toughness 8 (let's say it's an upright block of wood roughly 3 feet thick with some padding on it to take the abuse of training), the Shuriken would be DC15 + 1 Damage + 5 for three degrees of accuracy with Multiattack for a total of DC21. The GM would add the Toughness 8 to a resistance check vs DC21. If the die roll was 13 for a total roll of 21, that would be a tie and the dummy would take no Damage Conditions. The GM could decide that since it's inanimate,* it defends with routine checks (effectively an automatic roll of 10) and fails by 1 degree, taking a -1 Toughness penalty. The dummy rocks back slightly from the force of multiple hits as the stars bite through the padding and embed in the wood. Go pull 'em out.

Versus a sparring partner with Toughness 8 (Dodge 10 and Tou 8 is PL9 defensively, so let's assume it's a particularly tough martial artist working on his Ranged Defense skills), the Shuriken is DC15 + 1 Damage + no Multiattack bonus for one degree of accuracy = DC16. His +8 to resist would mean he can succeed with a die roll of 8 and take no Conditions. Again, since it's sparring, he takes a routine roll for a total roll of 18 vs DC16. He deftly rolls back to avoid the hail of shuriken but one sticks in his gi, although it doesn't break the skin underneath. He plucks it out absently, determined to improve his mobility and not get hit again.

Hope that gives you a feel for it.

*there's actually rules for Finishing Attacks on Defenseless targets including objects, but I won't go into it here.