I just signed up and started mining solo.ckpool.org with my bitaxe 401 supra. hashrate is around 625 GH. Since starting I have 670 shares with 9 rejected. This seems high as I have been told the reject rate should be below 1% ?
1% vs 1.3% isn't all that different is it? anyway, rejected shares mainly happen for two reasons:
1- Latency (internet-related issue), this is the most common cause for rejected shares, so check your internet connection, ping the pool with -t option in CMD, let it do 100-200 pings, and check the status.
2- Software/Miner issue: not very common with known ASICs since usually those software versions are heavily tested, whatever new small device you are using could be the issue, check with other folks if they have the same thing.
Also, just so you know, as far as I know, solo.ckpool.org has only one server and it's located in the U.S, so if you happen to be way to far from it, then you are more subject to high latency and thus more rejected shares.
With that said, since you can't do anything about it (unless improve your connection if it turns out to be the issue), I'd just settle with the 1-2% and leave it be.