I think the calculations could be improved. I have done similar things for myself and I can tell you what I did.
There are 3 costs associated with staff:
1. signing bonus
2. total wage he has received during the holding period
3. severance fee
What we want is to minimize the average weekly cost(awc) of our staff ([1+2+3]/no. of holding weeks). I determined the awc of my existing staff (including the signing bonus, which I think this calculator is missing) and the ones available in the staff market. Actually the awc changes every week. Initially it is usually very high due to the signing bonus. Then it comes down as the signing bonus is spread across the holding period and then reach a minimum(can be determined when 1,2,3 are available). It rises again because the staff is getting better pay. After reaching this minimum I compared this minimum awc with the awc of the staff available in the market. I would only replace my staff if I can find one with lower awc (in other words I am able to bid x amount to a particular staff since awc is related to 1, while 2 and 3 are constant for this particular staff I am bidding.) If no I wait and let the awc of my staff increases (it has already reached a minimum) until it reach the point where the awc of the available staff in the market is at least equal or lower than mine.