LSS = 38.94 * (T + A)^0.5
(Above should read the square root of T + A, but there is no way of putting in the root symbol!
Putting (T + A) to the power of '0.5' has the same effect.)
The temperature at 33000' should be -51 degrees. But since it is ISA +10, it is actually -41 degrees. Thus, assuming a lapse rate of 2 degrees per thousand feet, at 28000' the temperature should now be -31 degrees. This is now your value for 'A'.
T= 273 K and A= The actual OAT at your current FL
LSS = 38.94*(273 - 31)^ 0.5
LSS = 606 kts @ 28000'
By my reckoning... Do check, though! 