What mainspring for a nelson conversion lower?

I've been running my Nelson on a Sig 1911 lower customized by Dave Panciotti of Evolution Gunworks, he worked for both Sig and Colt Custom. He got my trigger down to about 2.75 pounds however he set it up to work on my 38 super so didn't mess with the mainspring. 

I put close to 5000 round through the nelson last year with nearly no issues. This year the trigger worked itself even lower and it started dropping the hammer to half cock. I adjusted the sear spring a couple times and fixed the issue however the pull is now 4 pounds, I could probably work the spring back down some without affecting function but not sure how far.

Since the Sig lower is dedicated to the Nelson should I consider a lower weight mainspring to enable me to get the pull lower without hammer falling?

I would look at a new sear spring, not a mainspring.  If everything worked fine last year it sounds like your sear spring is what has given up the fight.

Thanks KC. I'll grab one to mess with and keep my old one set as a backup.