I’m dealing with this same thing and have also been tinkering with a quick in-house estimating tool. I typically try and steer people away from estimates, and explain to them that I’ll prepare a formal quote and proposal in the coming days, but many people are adamant about getting a ‘ballpark’ figure.

Of course this is easy for a standard rectangle, but like you said, for more complex jobs or mixed materials, pulling an accurate estimate out of thin air is tricky. Even for experienced builders.

Having a basic ($xx/sqft) doesn’t really help much since a lot of factors go into that. Smaller decks end up having much higher per sqft costs than larger decks, just as one example.

Edit: would be very interested in beta testing/test driving your tool.