Dictionary:

ratable

  (rā'tə-bəl)

1. That can be rated, estimated, or appraised: ratable income.
2. Proportional.
3. Chiefly British. Liable to assessment; taxable.

Proportional; capable of estimation; taxable. A ratable estate is a taxable estate. In bankruptcy, a ratable distribution is a Pro Rata share of the bankrupt's assets. Ratable does not mean equal, but rather pro rata according to some measure fixing proportions.

That which can be appraised, assessed, or adjusted through the application of a formula or percentage.

Ratable property is that which is taxable or capable of being appraised or assessed.

The adjective has one meaning:

Meaning #1: (British) liable to payment of rates or taxes
  Synonym: rateable