Is "standard" really standard?

No, don't think I'd try and negotiate anything at this point.*

15% domestic, 20% foreign = standard.

* Unless the agency you're thinking of signing with uses non-standard royalty rates? I have actually heard of that, but it's rare. Victoria Strauss and Jim Macdonald would be two folks who would know a lot about this.

I think you're going to run into resistance at less than 15% for domestic. For foreign, there are enough good agents out there charging 20% that 25% is unnecessary, even for a first-time author. What's more, you should know that when they're asking 25%, what they're trying to do is make sure they're getting their straight 15% on every sale, anyplace in the world. That's because their foreign subagent is getting 10%. If the total tab to you, the author, is 20%, that means your U.S. agent is also getting 10%. I see no reason for you to accept the higher figure.

Standard

15% domestic and 20% foreign is standard. While a top writer can often get the 15% down to 10%, no new writer is going to do so. And you should never give more than 20% of foreign.