Which value? Do you specifically mean the -1.5? That's a value to counteract two things:

1) Potency doesn't scale at the value of the tooltip. My theory is that Each Point of Potency is actually a 0.01005 multiplier of damage, rather than it being Potency/100. So if a skill is 360 Potency, instead of it being a 3.6 multiplier, it's actually a 3.618 multiplier.
2) To accommodate for Rounding Error Values until I have enough data. So far, those damage models are the most accurate I've ever gotten them to be.

It was Viridiana's idea to convert it back to Potency/100, then resolve for the final value, otherwise, it would be (Potency*0.01005), and -1 at the end.

The Weapon Skill formulae is the Raw Damage at X Potency. If you wanted to calculate your DPS, instead of using the Potency of a skill, you would use the Potency Per Second (PPS) value of your rotation.

Auto-Attacks are the same. If you wanted to calculate DPS of Auto-Attacks, you'd use that formulae. If you wanted to calculate the raw damage per Auto-Attack, you would remove the (1/AA_DELAY).

If you're asking about the Division Coefficients, those are just scalers. Instead of multiplying it by a Decimal, you can inverse the operation by dividing it by a whole number.