This calculation is only correct if your volumes are correct.

And I don't think they are.

Something doesn't seem right with your measurements.

If your pre-boil gravity was 1.046 and volume was 7.5 gallons then you had: 46 x 7.5 = 345 total gravity points.

If you boiled off 1.5 gallons and ended up with 6 gallons of wort into the fermenter the OG should have been: 345/6 = 57.5 or ~ 1.058.

How sure are you about your volume measurements? If you really had 7.5 gallons at 1.046 then at 1.050 then you still had about 7 gallons of wort after the boil. For a 60 minute boil that's only a boil off rate of 6.67% per hour which doesn't seem likely.

I'm going to guess that your volume measurements are off and your SG readings are accurate. Also, are you accounting for the wort lost to trub and hops in the kettle? Any wort left behind in the kettle is also leaving sugars behind.