1.) Get a an adjustable protractor that has the bubble, then check for the angle of the car (rake) using something other than the roof. Check in 3 or 4 places to be sure. (often the recess that drops down from the fender into the engine bay is a good surface to use for this). Adjust the protractor until the bubble is centered and then record the number of degrees shown.

2.) Once you completed step #1, go on to check the engine position. Set the bubble protractor back on top of the carburetor / fuel injection and adjust the protractor until you have the bubble centered. The read the amount of degrees shown.

3.) Subtract the number found at step #2 from the number at step #1 and that is the angle your engine is actually mounted at. Most engine installations are angled down in the back by 3 degrees (to match the rear end pointing up by the same 3 degrees) 3 degrees is the goal but many installs run with more than 3 with no issues.