Trying to count using where with child and linked tables data

Hi,  I am new here and have spent a long time trying to do a simple count count formula.

I can use cnt(Observations.Regulation.Code) to easily count all the codes on the form, but what I want to do is something like cnt(Observations.Regulation.Code where code = "C1") so that it only counts codes with C1 in this record.

The stucture is quite basic, but I just cant seem to get the where count to work, tried lots of various select statements but I can only get this to return information from the Regulation table for example. cnt(select Regulation where Code = 1) counts all the records in the Regulation table, but I cant relate this back to the current Report table.

Table - Report
   child - Obb

Table - Regulation - Linked to Obb