Bell Curve Help - Calling All Stat Monkeys!!

So, I am able to accurately create a bell curve and allocate 100% of the costs under the following simple scenario (whoopie):

Total Costs: $3m
Month Start: 1
Month End: 12

However, when costs to date come into play (as one lump sum), I need to curve the remaining costs rather than the full budget. The formula that I use in the above scenario gives me the correct shape, but the allocation is less than 100%. Which makes sense, because I am only looking at part of the bell curve... but, I need to figure out how to get it to 100% (example of assumptions are below):

Costs to Date: $1m
Costs Remaining: $2m
Total Costs: $3m
Month Start: 1
Month End: 12

There is something wrong, and I think it has to do with my costs to date being one lump sum and my curve not accounting for the costs spent to date or the shape in which they were spent. The problem is, I only have access to this lump sum number (lazy accounting team where I work). I am using the normSdist function and my zscore is the relationship between the period of the analysis and the total time it takes to spend the costs...if that helps. Any guidance?