You can do it by adding a second level of Select
:
Projects = group.Select(pd => new
{ // This level deals with the first level of computation
Count = group.Count(),
CostToComplete = group.Sum(pdd => pdd.totals.costToComplete),
TimeWorkable = new UserBillingRate(WS, group.Key.manager, new Period(start, end)).TimeWorkable
// This level takes the computed results, and add derived computations
}).Select(pd => new {
pd.Count,
pd.CostToComplete,
pd.TimeWorkable,
// Now that pd is an anonymous class created by the level above,
// both pd.CostToComplete and pd.TimeWorkable are defined.
StatusOfCharge = pd.CostToComplete / pd.TimeWorkable
})