Hi Guys
There is no hard and fast mathematical solution to this at all i'm afraid.
Each Bb as you know has a different set up and as such will need a different length of mouthpipe,Bell and slides to get her into tune and enable the slotting to be right.
I usualy start with taking around 8% of length from the horn as a point of reference, then its the old bit by bit method on each part to get it correct.
It may well end up that you have in fact taken a total of 12% off, but i like to start small and have the safety net of knowing that i have enough tubing to work with.
Of course once you have done this to a certain model then you have a reference point for then next same model! as long as you have taken down the measurements of what you have done of course
, which i have forgotten to do on a couple of occasions.
This is ONLY a reference point though as even the same models of Bb can vary slightly, so always be very careful to not dive in with the exact measurements of the last correct conversion.
REMEMBER !!! taking another mm off when you are still too flat is an awful lot easier than adding tubing/length when you have been over anxious to get the job done by taking far too much off.
Of course you can always pull the tuning slide out further to compensate for it being too sharp, but you don't have too much length on a C to mess about with before it starts to throw out the intonation.
Slowly does it every time in my opinion
I am sorry to tell you that there is no other easy solution to this.
Once you have done a few you will start to get a feel for what is needed to balance the horn off.
Best Wishes
Leigh