How accurate is the Kaspa solo mining calculator?

The solo mining calculator gives you a long-run average, not a guarantee for any single block. Because Kaspa's block discovery is a random process, the actual time between the blocks you find can be much shorter or much longer than the number the calculator shows. The average only becomes meaningful after you have found several dozen blocks — that is the observation window over which the statistics smooth out. For a beginner, this means treat the calculator as a planning tool, not a promise: real payouts will vary unpredictably around that average, especially early on when your sample size is small.