If you’re in doubt, I’d let your company know and get written confirmation that they allow you to do this on your own time before trying to sell it/use it for your work.

If you just made it and it doesn’t directly compete with anything your company is doing, odds are that they won’t care. Most companies aren’t out to steal your ideas and the general clause about IP is for avoidance of doubt and to deter employees using their knowledge to compete.

If it turns out you’re making millions from it and then you ask, they might get petty and claim it, though. With written approval beforehand, you’ve got your legal precedent in order.

Beware that they may tell you that they’d prefer you not working on a second business simultaneously.

How do you assess pre-revenue companies? Do you just look at other growth metrics in the model, or do you not invest in companies with free products?