Converting Loran TD's to lat/lon in my GPS - offset values?

I have a list of Loran TD's that I want to convert to lat/lon.



I have read all about Loran, and I know that this is not a simple conversion and that it won't likely be perfect. But I want to get as close as I can.



My Garmin GPS allows me to enter data in TD format, which is how I plan to do it.



I figured out what Loran Chain was used to capture these spots, and which two stations were used. Chain 7980, stations W and X.



The question I have is about something my Garmin GPS is calling "TD Offset" for each of the two stations I am using. The Garmin is asking for a decimal number, and is defaulted to 0.0. I don't know what to use here.



I have the both the Loran TD's and lat/lon numbers for a couple of spots that I think I can use as a reference. When I enter and convert the Loran TD's, they are very close to the lat/lon but not exact. Do I need to just start making guesses with the "offset" until the resulting lat/lon matches? Is there a more scientific way to do this?



Any thoughts?



Thanks in advance.