How much power is lost through a 518

You will need to clarify what you are talking about here .
Here ya go
To find your cruise timing at that rpm; (edit; minimum cruising rpm), just keep tugging on the timing, while simultaneously reducing the rpm back down to the minimum cruising rpm, until the engine no longer builds rpm. Edit; I use the fast idle cam to set the minimum cruising rpm.So for example; you add 3 degrees timing and the rpm goes up say 200rpm. You back it down the 200 and add 3 more degrees. This time it might only rise 100rpm. So you back it down, and add 3 more and this time it goes up only barely, so you take the last 3 degrees out and call it done. End Edit.