I don't have a RO-Jett .61, but I'd suggest setting the pipe length at whatever Randy's chart suggests for the exhaust timing it has...which I believe is up around 145*. Then, I'd borrow a CF prop or two from your locals and start launching at 10k to 10.5k. Using a prop you can't replace or isn't potentially the best prop for the combo isn't very productive, IMO.
Always get a reading on launch rpm every flight, so you know where to go from there. Be sure the engine is fully warmed up, because if it isn't, you don't really have a baseline. The engine will lean out as it heats up...about the time you get the handle strapped on. Don't run the pipe shorter than suggested...tends to burn down the engine, I think. I've heard PW say that a couple times, not speaking to me, however. Brian Eather makes really nice stunt props, and is pretty quick to get them to your door.
Email Brett if you need
good prop advise. I would guess wrong, no doubt, but I've had very good results on the .46VF and PA .51 with the 12 x 3.75 undercambered 3 bladers (full 12" for the .51, 11.5" for the VF), so I'd probably try a 13" version and pitch it to 4" and maybe pitch up the last 2" to 4.5" or so. My experience indicates that the U/C props work better on overweight models...or maybe for overweight pilots? YMMV~
Steve