You have a poor fact table design, if you cannot get something as basic as the sum or the orders. Your so-called "OrderFact" is really a "LineItemFact". If it were an "order fact", a given order would only be on one row.
If this were designed as a LineItemFact, then the tax would be on a separate row and you could just do:
select sum(LineItemTotal)
from OrderFact
Absent that, you could have a "LineItemNumber to enumerate the rows for each order. Then you could do:
select sum(OrderTotal)
from OrderFact
where LineItemNumber = 1;
With your structure, you can do:
select sum(OrderTotal)
from (select OrderId, max(OrderTotal) as OrderTotal
from OrderFact
group by OrderId
) o