1 1/4 inch for the 370 gr bullet.
The 400360 BACO mould shoots well in the 20 twist 40’s

Presuming you may have a 18 twist in that 40-90bn
use a 1.25" long bullet.
The Greenhill formula with the constant of 150
does not work for our velocity range.  Use a 120-125 constant
beltfed/arnie

Well, Dave, my 18 twist 40-60 Maynard would not perform at distance with my 410 grain bullet. I talked with Dan Theodore about it and he said, no way, that I must try a 370 grain bullet. I did and boy was he right.

For my .40-60 Maynard using a 410 grain Money  1.433" versus 422 grain Creedmoor 1.366".

Caliber      0.408      Inches            
Bullet Weight      410      Grains            
Bullet Length      1.433      Inches            
Barrel Twist      18      Inches/turn            
muzzle velocity      1250      fps            
Temperature      50      degrees Fahrenheit (59 is standard)            
Pressure      29.92      inches of mercury (29.92 is standard)            
                       
Sg =      1.49            

Caliber      0.408      Inches            
Bullet Weight      422      Grains            
Bullet Length      1.366      Inches            
Barrel Twist      18      Inches/turn            
muzzle velocity      1250      fps            
Temperature      50      degrees Fahrenheit (59 is standard)            
Pressure      29.92      inches of mercury (29.92 is standard)            
                       
Sg =      1.76                  
                       
Both bullets provided good accuracy out to 400 yards @ 1,200 fps. However the pointer Money bullet would loose accuracy around 400 yards and had poor accuracy on the Rams. This was corrected by increasing the velocity to 1,250 fps to stabilize it's yaw over a longer distance. The pointier Money bullet has the center of gravity further back resulting in more yaw which needs more velocity to stabilize that yaw to shoot longer distances. The problem here is not the length of the bullet but the location of the center of gravity (which changes with nose shape).  The Creedmore bullet was stable at long distances even at 1,200 fps because its center of gravity is further forward which reduces the angle of yaw so it requires less velocity to stabilze the yaw at longer distances.      

So these length bullets are quite appropriate in a 18:1 twist barrels but bullet shape results in different center of gravity positions which change the bullet's yaw as well as the minimum velocities required to stabilize that angle of yaw. Yaw stabilization (not gyroscopic stabilization) is absolutely crucial for LONG RANGE long stability and accuracy.

Of coarse you first must obtain gyroscopic stability first at short range but long range stability requires obtaining dynamic stability which is all about the center of gravity of the bullet and the minimum velocities required to sustain that yaw angle.

Or you can do as Dan suggested and shoot shorter bullets at greater velocities instead of finding a bullet with a better center of gravity with the velocity fine tuned to stabilize your yaw. This approach precludes you from the benefits of achieving better kinetic energy from shooting heavier bullets which allow you to shoot longer distances and be less susceptible to wind drift.