You can assume that honey will have a potential gravity of about 1.035. That means 1 lb in 1 gallon gets you 1.035. There might be better numbers available if you've measured your honey, but it's an approximation. We'll call this 35 gravity points.

So, you added 1.5 gallons, that gets you 52.5 gravity points. If this is a 5 gallon batch, then you just divide by 5 to get 10.5 gravity points. You're starting gravity was 1.050, or 50 gravity points, so 50+10.5=60.5. So once you added the honey you can estimate a SG of 1.0605.