I am no Paul Sellers. But as Chris Schwarz says "even a gappy dovetail is a really strong joint". Two joints in I wised up and did the blue tape layout trick, that helped a lot. For strength, you almost always want the tails on the sides and the pins up. The reason is if the glue fails, the mechanics of the joint still. only allow the side to pul lout sideways not up/down. Since almost all of the force on any kind of casework is up/.down, that's what you go with. SO in the phot below, the top is actually vertical with the side sticking out the back.
For spacing I eyeball it. When I have less massive pieces of wood, I always cut the tails together two sides at a time, then cut the pins to match. Since you're cutting the pins of off the tails, the layout is surprisingly forgiving as long as the tail cut is square and doesn't go below the scribe line.
The trick with large casework dovetails is that it's slow to cut em by hand. The main thing is to make sure your saw is actually sharp and cutting straight. About halfway through this set, I also rigged up a bandsaw sloped ramp rig I can use but this case was too wide to fit all the way so I ended up with about 3/4 hand, 1/4 bandsaw on the cuts. I also own one of the fancy Leigh dovetail jigs, but honestly I hate routers, so I use a lot of hand tools. I wouldn't recommend doing the joint without a good bench, saw and at least one good chisel.
I did the sawdust/ glue trick for gap filling, but i think the sawdust / shellac trick will be in my future. I have enough board left over for a matching BFPR head cab assuming I can get the reverb tank in there compactly enough.
The dovetails can look pretty rough when they come straight off the saw. The main failure is actually the board cupping across the width such that the middle of the joint is loose and the sides are tight. Mostly just practice. There's about a million and three "how to DT" videos on you tube, Rob Cosman is good, but he seems to think it's a lot easier than it is. One other huge danger is cutting the joint backwards or inside out. I now use a hammered stamp to mark my joints on the inner corners A-B-C-D, etc. I found I was sanding off the pencil marks too much.
We build because we must.
