RPA (rocket propulsion analysis) is what I use. The free version (1.2.9) has basically all you'd ever need, version 2 limits the number of propellant components but does nozzle geometry and heat transfer, so I recommend 1.2.9 for the most part. If you mean modeling an entire engine burn, there are a couple hybrid spreadsheets floating around and then HRAP (hybrid rocket analysis program) which I developed and posted to GitHub.
Are you using the 'stock motor' or with changes / improvements?
Mostly stock aside from the pre-heater (probably along the lines of the new Contrail "Ring of Fire" igniters), I got Dave to make a reload for it a year or so ago. Even stock this rocket is simming to ~12K feet, which is well over half the max altitude any of my local fields could accommodate. Planning to re-fly later on a swirling injector with a shorter ABS grain, which should get it to around a 50% L350, but that'll require a trip out west.
If you just change the injector to the RattworksXFlow injector, it becomes a K400 ish. Much more efficient.
Or you coould use brass printer nozzles or gas jet nozzles. Brass printer 0.2 mm M6 thread would be 0.01" ish and not $11 each ...
I can measure the rattworks 29 and 64. Once i can get my hands on my acetylene nozzle cleaners to get a size and find my vermiers. I think Contrail mentioned their vent size before, here. Maybe we should set up a hybrid dropbox and have a single source for useful info.
I got you beat, I can drill 100 vent fitting hols and break many bits....
Stop hand drilling them......
. Printer nozzles look like a good easy option to me.
Have you read the K240 injector tests? Attached.
I have, I use the standard grain with the x-flow injector
I'll burn my last original (PVC) grain first and I have an optimized ABS grain for another flight.
M5 and 1/16 npt pretty much fit each other....I just got my West Coast Hybrids I110 ready for the weekend and it was also missing the vent but I had a bunch of idle jets left over from tuning my motorbike. Made an adapter from 1/8" NPT to M5 thread, one side gets the jet (0,42mm) the other a small fitting for 3mm PA hoses (roughly 1/8). Can't wait to fly it
Not really, 1/16 has an OD of 7,89mm and a pitch of 0,941mm vs 5mm and 0,8mm for M5. The threads on the WCH parts are 1/8" and the fittings for 5/32" / 4mm fill hose have the right ID so you can just tap a M5 thread in them.
Might be ok but.....
You're just not using enough teflon tape Mike......
Now you're thinkin to yerself, "did he tap this 1/4 BSP or 1/4 NTP..... Do ya feel lucky?"
Also the vent restriction is not just about the diameter, it's about the length it applies.
in the brass NPT plugs. the .028 hole is about .110 'long'. Same as the OEM Skyripper.
the 1/8" --> M5 adapter is tapered, there is a aluminium washer on the idle jet has and the 1/8" hose fitting comes with an O-Ring.
flew the 54mm prefilled motor with the bigger tank to 1443m and the WCH I110 one time on PVC (1073m) and once on a more complex ABS grain (only 853m, same rocket). All flights were successful but the lower altitude on the WCH ABS flight means I have to put it on the teststand... Oh no 
Yes, it's safe. But it's going to be messier......
