Market appreciation does add credit to a Reg T account's SMA. At the end of the regular trading day (normally 15:50 US/Eastern), there is a check for the amount of the Reg T Excess (Reg T Excess equals Reg T Equity less Reg T Margin). If the Reg T Excess of a margin account is greater than the previous end of day Reg T Excess then the account receives a credit and SMA and is set equal to the new amount of Reg T Excess. The change to SMA resulting from trades is effectively the change in Reg T Equity minus the change in Reg T Margin.
Please note that an SMA balance will never decrease because of market depreciation of the open positions. Because SMA does not decrease from market depreciation and stays at the last "high water mark", any end of day market appreciation will only be added to SMA if Reg T Equity is currently higher than Reg T Margin.
Additional Information