RFC-120 - FTL "This is the ideal time" is not justified with a reference

[stringhandler]

In this RFC, the FTL section has this statement:

The FTL is calculated as (T*N)/20 with T and N defined as: T: Target time - This is the ideal time that should pass between blocks that have been mined. N: Block window - This is the number of blocks used when calculating difficulty adjustment

I'd like to know what the basis is for this being the ideal time. It is currently 9 minutes, which is 4.5 blocks, which would mean that N is 90, which seems ok, so I would want to know where the formula comes from