I have always found the Moroso Power/Speed Calculator pretty much dead nuts accurate, IF the car is well sorted out. My Fairmont and Mustang both line up ET vs MPH wise perfectly, and although the Mustang is pretty well perfectly aligned with HP numbers as well, my Fairmont actually shows the engine makes a bit more HP on the slide rule than my dyno sheets show.
Most typical street/strip cars are at least 1/2 second slower ET wise, compared to their trap speed. And even being off by more than a full second is not unusual for a miloly geared car with street tires.
Now, whenever I tried some common website supplied computer calculaters , they were off by a mile. Some showed my Fairmont as having over 700 HP, about 150 more than it actually does. Also, the trap MPH is a much better HP indicator than ET. If I stuck a pair of 195 75R14 street tires on my Fairmont, it would likely still trap around 130 MPH, (if it didn`t crash!), but the ET and especially the 60 foot numbers would be absolutely terrible.