Beambot has the source. The general idea is its ok to lose Money if your net (growth minus losses) is higher than your cost of capital. (VC money expects a 30-50% rate of return) In general SaaS companies have high fixed costs so it’s a race to get big enough to cover them. The rule of 40 flags companies that are too unprofitable or not growing fast enough.

Like all financial metrics (PE ratio, etc) it’s still just a crude guide.