I vote for +/- 15 degrees
You are slowing down when you start to use increased effort and therefore angle on the rudder to counteract weather helm from the heel. You can check this with your knotmeter on a windy day. When you're powered up enough to heel around/over 15 degrees and start putting moderate effort into the helm, drop the main on the traveller far enough to reduce heel while watching your knotmeter. On my '84 E26, once the heel was reduced, the rudder goes back to near center, the effort goes down and I gained a tenth or two of a knot from reduced rudder drag.