GST Rule amendments revise refund calculation, valuation deeming, fund deposit requirement, and enforcement measures. The rules prescribe that the value of supplies added under valuation provisions is deemed paid for input tax credit purposes; extend a prescribed time limit to eighteen months; and set out a formula for refunds on account of inverted duty structure where Maximum Refund Amount = (Turnover of inverted rated supplies x Net ITC / Adjusted Total Turnover) - tax payable on such supplies, with Net ITC and Adjusted Total Turnover defined, and require inward supplies to be received against a tax invoice for relevant claims.