For some reason I read your post as if you were going to take your boat out, rather than launch it. Regardless, the process is similar, just in reverse order. Perhaps this is one of those "easier done than said" things but I'll give it a go:
The slings at the Port of Edmonds launch are 8.5 feet on center and they are fixed so they cannot be adjusted. The cutouts on the bunks of our 2021 trailer are wider than that (maybe around 10 feet OC). The bunk cutouts are needed because when the boat is set down on the bunks, the sling straps nest in the cutouts. That allows them to be pulled out from under the boat, otherwise, the hull would sit on the slings.
Your boat should have two stickers (marked "sling") on each side of the outside hull just under the gunnel that identifies the factory designated sling points. If you have a dark blue hull like we do, they can be hard to see. Those stickers line up with the bunk cutouts on your trailer.
The problem is that Edmonds' sling straps can only line up with one of those stickers, so the other strap will be short of the bunk cutout when your boat is slung. If you're launching, the sling operator has a choice of threading the sling strap through the rear cutout or the front cutout, but not both on our stock trailer. Their solution has been to use the front cutout to lift the bow off the bunk, block the bow up, slide the sling back to the point they can get the rear strap through the rear cutout (the front strap just slides back between the hull and bunk). They can now lift the boat off of the bow block and trailer to launch it. If you're retrieving, they'll submerge the sling straps and you float the boat over the straps. If you line the rear sling strap with the aft "sling" sticker, that strap will land in the rear trailer cutout. However, the front strap will be short of the cutout and would get sandwiched between the bunk and hull. The operator will put a support block under your bow before the bow lands of the bunk, allowing them to pull the strap out. The stern of the boat will be sitting on the trailer. They pull out the rear strap, move the front strap to the align with the front cutout, raise the bow off the block and remove it, then lower the bow onto the bunk. The front strap can now slide out through the front cutout.
All of the blocking/moving is why they charge more to launch boats with bunks like ours. From what I've seen, the sling operators are very experienced in doing this and have a process. When I made new front cutouts to align with their sling, it eliminated all of that. They just slide the straps under and lift the boat or set the boat down and pull the straps.
Disclaimer: I chose to keep the rear factory sling point because I believe there is more boat-weight aft. However, I did not consult with Fluid Motion to ensure that moving the front sling point rearward a few feet was a proper sling location. So far, so good and I don't foresee there being any issue. You should consult with FM if you intend to move your sling location.
More launching info and a graphic of the Edmonds sling dimensions can be found here:
https://portofedmonds.gov/marina/boat-launch/
