General rule of thumb is that at our trap speeds and car size, 1 car length is approximately 1/10th. That's always worked for me anyway, but I've never done the actual math to verify it. I just trusted the more knowledgeable people who passed it along to me. So lets see what happens:
WRX length = 178.5" or 14.85 feet.
A car travelling at 100 mph is covering 1.666 miles per minute (100 divided by 60)
which is .0277 miles per second (1.666 divided by 60)
which is 146.6625 feet per second (.0277 times 5280)
which is 14.66 feet per 10th of a second.
So, that settles it for me now!
I also go by 10 hp being worth 1 mph in trapspeeds, but that's only from experience and seeing other cars. Not sure if there's a good formula for that, and it's too late for me to try.