Baldry's rule establishes the theoretical pivot centre for a pony truck such that the pony axle will be radial to a curve.
Data has been taken from 'Building Model Locomotives' by Roche and Templer and corrected.
The position of the pony truck pivot can be found by graphical means or by calculation. You will note that the track radius is not involved. The result is correct for any practical radius.
Then establish point B at the centre of the fixed wheelbase such that AB = BC.
Then draw line EG at right angles to AE and of length equal to half the fixed wheelbase, that is EG = BC.
Now join BG and bisect it to find point F where BF = FG.
Now draw FD at right angles to BG and intersecting the line AE at D.
D is the pivot point, the pony truck radius arm will have length DE.
Formula is Y = (T-X²/T)/2
Therefore X = 8.25, T = 17.25
and Y = (17.25 - 68.06/17.25)/2 = 6.65
Baldry's rule puts the pony axle radial to the curve but is based on pure geometry. There is no allowance for sideplay in the axleboxes or between wheel and rail. To compensate for sideplay the pony radius, Y, should be increased. Otherwise the pony will turn too far and try to climb the inner rail. Compared to the prototype our models have exaggerated sideplay so more compensation is needed. So if you run on sharp curves and have sideplay to suit, you may need to try a longer pony radius, experimentation will be needed.