Tail sitter roll offset

Hello all, I’ve just built a tail sitter with two motors and I am experiencing the following problem: when the plane is ready to take off in vertical position, i can notice a little angle in the roll axis on the left (something.like -1.8 degrees). This, when I apply throttle causes more power on the left motor mix which translated onto right drifting.

I noticed that if I correct the position of the sensor to have roll zero reading before takeoff the problem disappears. Now the question is is there a parameter I can set.to.clmoensate for this little error on the roll (in Q.mode) orientation of sensor ?

thank in advance!