This seems like a less useful version of FireSim calculator, which I already take a huge issue with when people nitpick about 90% success vs 100% success.
As you pointed out, your estimates already have padding, which means they're fundamentally a very, very rough estimate of your likely spending.
If your own estimates are rough, then how can a calculator be accurate down to a few percentage points.
Let's not forget that no calculator can predict the future.
So no, I wouldn't give this extremely rough calculator much credence at all beyond the vaguest of guidance.