For this type of requirement I like to add a "Pre " prefix to the variable so that I know immediately that a calculation will be made using this field.
Keeping this in mind, let's rename your initial field to "Pre List Assy P/N".
Now create a calculated field named "List Assy P/N" and give it this formula:
[font="courier"]If(Left(,1)="B",Substr(,2,17),[Pre List Assy P/N]) /font[/quote]Which basically says "if the first character of the field is "B", then use only the remainder of the field, otherwise it isn't "B", so use the whole field. Assuming that your value of 17 is large enough. You can CYA by increasing the 17 to something absurd like 100 and Monarch won't complain and you'll never have to revise the formula if the original field expands.
Edited to add forgotten ")" in the formula.
Just as an aside, instead of having to try to figure out if you need to use 17, 100, or 1000 characters, the formula can be adjusted as such and never touched again:
[font="courier"]If(Left(,1)="B",Substr(,2,Len()),[Pre List Assy P/N]) /font[/quote]The LEN() formula will always adjust to the length of your field.
Just my 2c worth!