Does using "High Leg Delta" 3-phase electricity require a different equation for calculating Amperage/Power?
Does using “High Leg Delta” 3-phase electricity require a different
  equation for calculating Amperage/Power?


No
Current and power are measured independently of neutral or ground on High-Leg Delta.
However, transformer supplier may limit load imbalance.


  should the same equation be used to anticipate required amperage?


Yes

If imbalance is specified not to exceed x% then differential current protection is required.

If power up surge is grossly imbalanced and caused low load phases to run over voltage then maintenance must improve load balance to prevent power up unbalanced surge voltages.