Probably true race guys are usually shotting for a certain et time and or it's a continuous progress, street guys when building an engine are really not shooting for any performance goal so it becomes I want 400, 500, 600 hp etc... whatever they think will satisfy them usually eyes bigger than belly syndrome. I think anyone building a car should shoot for a certain et goal even if it's never gonna be raced gives your project focus and everyone wants a certain performance ability even if it's like I'm ok with 16, 15, 14 second et gives you some foundation of what parts to use that will work well together.
As for the HP calculator, I just use the 1.4 cfm per hp rule of thumb and the cid x rpm/3456 = cfm, to get ballpark. So if I wanted a 400 hp engine that peaks around 5000 rpm would be 400 x 1.4 = 560 cfm so 560 x 3456 / 5000 = 387 cid.
