The actual equation to use is this one:

S1*V1 + S2*V2 = St*Vt, where S1 is the specific gravity of the first liquid, S2 is the gravity of the second, the V's are the volumes of each, and St and Vt are the net specific gravity and volume of the total.

So, for your pear juice, S1 = 1.053 and V1 = 0.875 gallons (remember for a one gallon total batch some of the volume will be from your honey addition). Adding 1.5 lbs of honey (roughly 2 cups, or 1/8 of a gallon), S2 = 1.417 and V2 = 0.125.

Then 1.053*0.875 + 1.417*0.125 = St*1.0, or,

St = 1.099 (close enough to 1.100).

Your calculation didn't take into account the difference in volume once you add the honey. That's one of the reasons that I use Hightest's spreadsheet these days - I always make simple mistakes like that when I try to do the calculations by hand.