How do I account for a LOC used for down payment

[Greg Smith]

I am planning to use a line of credit against a house I currently own to fund my down payment toward buying a new SFH (pulling out the equity from one house to buy a different house). It is pretty easy to estimate interest... what I'm wondering:

When calculating my CoCROI, should I use the amount I'm pulling from the other house as my cash in number?  I figure that's fair because I'm could use that for any other deal as well.  It seems not quite right though because it's borrowed.