Adaptive Modulation and Packet error rates

Hi,

There is a formula that gives you the amount of data that can be
transmitted on a sub-carrier when the SNR is known and the target BER is
also known. It is something like this

c(n)=log(1+(-1.5/ln(BER))*SNR)

However in real applications packets are used and target PER's. I have
found a relationship between PER and BER and it goes something like this:

PER=1-(1-BER)^L


L:length of packet

So... is this how i am supposed to make the conversion to use the adaptive
modulation formula? What would typical values of L be? Any advice on this
would be most useful