Here are your mono spool yard capacities / lb. test for your DM20A: 290/14, 230/18, 210/20
Next thing you need to decide on is how deep do you want to troll with your leadcore? On average – one color (30 feet) will get you a depth of 5 feet trolled at 2.0 mph. More times than not – 6 colors (180 feet of leadcore) will get you down to 30 feet and that should be plenty. If not, adjust accordingly.
The goal is to fill your spool up because your line counter reel reads revolutions of your spool and not actual feet. In other words, if your line counter spool is half full – it may be reading 13-18 inches per revolution versus 12 and you’ll be off up to 33% when you fish.
I like putting on heavy cheap mono – 20 lb test for my backing to help fill up that spool and then add my leadcore line.
So…. now I’ll answer your question. Since your spool can accomodate approx. 210 yards (630 feet) of 20 lb test line, reset your clicker to zero and put on 400 feet of mono (your clicker should say 600 when you’re done because you’re putting it on vs taking it off). Reset your line counter again to zero and now add your 6 colors of leadcore (180 feet) or until your spool is full.