Gears, sprockets and chainwheels...Oh, my!

[85RZwade]

I'm having one of those moments when I realize I don't know the question to ask, much less the answer. I gotta acquire a rear sprocket for my RD350B project, but how many teeth (OK, I knew that question)?
The known values:
'75 RD350 primary gears
'76 RD400 transmission
520 chain
TDR250 rear hub, same sprocket as FZR400 (520)
The 400 countershaft is the same spline and size as my RZ and TZ, so I have a couple of front sprockets to play with, but I have to buy a rear. Is there a way to make an educated guess, or have I entered the realm of the experimental?

[DesmoDrew]

https://www.gearingcommander.com

[85RZwade]

That's a lot for my tiny reptilian brain to absorb, but I think I see the vision. It seems as though I need to know what final drive ratio is my end goal, then plug in known values and manipulate sprocket sizes to reach the goal ratio. Therefore, I need to know what ratio is desirable, which is probably fairly universal. Any two-stroke twin streetbike with a ratio that is satisfactory, such as my own RZ350, could be used, right? Determine the RZ's ratio, plug that into the gearing commander with my other known values and I should be able to derive a couple of workable sprocket combinations.
Thanks for coming along for the ride while I worked that out, and please shout out if I've slipped off the groove on this one.

[RDFL]

https://www.gearingcommander.com

Great link

[SoCal250]

https://www.gearingcommander.com

Great link

Agreed!
Thanks Drew, that site is awesome. I just spent a whole bunch of time plugging in numbers and tweaking for my RD125. Great info.

Wade, when you run your calculations be sure to double check the preloaded values for the bike selected because I found that many numbers were off a little. especially the transmission ratios. 

[85RZwade]

Thanks, I'll check them against the service data