OK, I'll talk a little bit about it and the thought process.
At 550-600 rwhp you're kinda on the verge of being able to hang 4th gear. It will depend a lot on the surface, the sled, the track length, and whether or not you can run front hanging weight.
If the sled lets you run 290', you won't have much time to hang 4th and really sail. So you need to run 3rd low and "bulldog" the sled, let the motor wind out as best you can. In this situation you do not want ANY wheel slip. 100% traction is what you're looking for. The best advice I got was from Kyle Michael - 45 rear, 30 front and work your way down until she hops. I found that anything below 38 in the rear would get bouncy, but in between those numbers, I could find fantastic traction with the 35" STS. I've run the fronts as low as 18, but in the mid 20s seems to work fine for most tracks.
I embarrassed the hell out of myself twice trying to run the recipe that some people use - full hard in the rear (65 psi) and half that in the front (32ish). Terrible...motor would get up on the charger and just absolutely annihilate the tires. Just awful. Now, had I had more cojones, when that happened, I should have tried to shift her to 4th....but most of the time I had my CoPilot set on kill and it would have snuffed me out with the locked 3-4 upshift. If the CoPilot was immediately changeable (or if I had put in the tricky switch) then you can kinda "slide" through the 3-4 without the violent and immediate 40% rpm drop. But live and learn.
Now, for the roadrunner sleds, if you go 3rd gear, you won't get the speed you need to play the inertia game at the end when the pan drops. Kyle used to tell me that he loved the hard hitting sleds because up until then, you can run like hell. So this is where you need the wheelspeed. And 100% traction will likely kill you - you need some slip to keep the RPMs up to make it though the 3-4 upshift and TC lock. So up we go with the pressures to something like say 48 and 27 (or higher; 50/30 is not bad) and you get the slip you need. When you do it just right, the truck feels like it's on a gumband....motor whizzes up and the wheels start churnin' real hard, and you think you're going to do a sit 'n spin, but doggone, it just catches up and takes off like a stick truck.
This summer, I had been convinced by a few folks that when running a good amount of front hanging weight, that I "should" be able to come up with the pressures a bit and do better with more ground speed. Well, make a long story short, I ended up doing a lot of spinning. When I finally got my head wired back up to my ass, I went back to down to the "sweet spot" where I had learned to run the truck with no FHW, and man she came right back and felt a whole lot better. I was kinda miffed because I felt like I left a lot on the table at some hooks, but again, I learned something.
Now at 800 hp or thereabouts, you need 4th gear, period, and getting that fine balance of slip out of the hole and grip at the big end is everything. I think Tomac had this mastered first and then guys like Scrimager and Burkhart came along and proved without a doubt that it works.
Hope this makes sense. You have to know what your truck can do, then go look at the track and sled, and adapt your pressures and driving style to suit. There is no universal recipe!
One common error that I see all the time, is guys BSing and fiddling with their stuff, instead of being out on the track where they should be.
Best advice is, have your truck 100% squared away when you get there, so you can spend more time on the dirt.