The best estimate is around 10%, or slightly rounded from there. In your case, it would be 18lbs, so you could do either 15lb or 20lb jumps, earlier in the program to get a feel for heavier weight, I would do the smaller jumps, but that's up to you.
So your sets would look like:
120x5, 135x5, 150x5, 165x5, 180x5
Or:
100x5, 120x5, 140x5, 160x5, 180x5
Good luck...