Summation Part of Erlang C Formula

[tinydancer]

What would be the best way to write the below summation into a formula in excel?

It is part of the Erlang C formula and I am looking to calculate it a number of times. Right now I have a table that changes based on the "A" value then I use a vlookup based on "N-1" to go grab that sum. I can't build a whole table each time for a calculation I'm looking to do well over a thousand times. Looking to see if anybody has a workaround to turn this into a formula.

Thank you

 

[tinydancer]

Thank you for the help Eric W and bobsan42. This worked exactly as I needed.

 

[bobsan42]

Glad we could help.
Good night. ?

 

[shg]

I have a formula for Erlang C that will work for large numbers of agents if you need it.

 

[tinydancer]

I have a formula for Erlang C that will work for large numbers of agents if you need it.

If you're willing to share I'd love to see.

 

[shg]

For call intensity u (arrival rate times duration) and m agents, the probability of waiting (Erlang C) is

= 1 / (1 + (1 - u / m) * EXP(GAMMALN(m + 1) - m * LN(u) + u) * POISSON(m - 1, u, TRUE))​

Very pretty, huh? Credit for the math goes to user Dap at StackExchange in this thread. He/she commented that Excel doesn't give accurate results due to the POISSON function implementation, and while that is indeed true in Excel 2003, in Excel 2010 and onward, it's fine (I didn't have Excel 2007 to check).

Please acknowledge Dap and link the thread if you use his/her formulation.