that is the pipe that i thought you were speaking of. just could not think of the name mini pipe. the timing of the return pulse is multifold and works generally only for a very small rpm range. it can be adjusted just like the full tuned pipe but as i said generally it is designed for the higher rpms.
first select the prop you want to use along with the fuel that you plan to run. keeping the same fuel is critical as the flame front pulse changes from brand to brand, oil mix, and nitro content.
run engine without any muffler, port adapters etc. peak engine and get rpm. write it down as your base referance.
install mini pipe re-run engine get rpm (most likely rpm will drop significantly)
this is where it gets tedious and can get expensive
1 inch at a time lengthen distance from center of glow plug following the curve of the adapter to the muffler. it may take 5-8 inches to get the rpm change into the correct area.
the correct area is determined by a rise in rpm, flattening out and then dropping again.
once it drops start shortning the extension by 1/8 inch at a time till it peaks. this is the sweet spot for that fuel, day, engine, glow plug (yes they do make a major difference)
all of this testing needs to be done in a fairly short period of time as all weather variables combine to screw things up.

many years ago i was a sponsered r/c boat racer with full tuned pipes runnng 70%nitro at 30k rpm+ using ops engines up at a rate of 4 or 5 engines in a day. (do we say pushed to the max)
good luck i hope you can make the adjustments work at those low rpms.