If you do select count(*) from tblPhoneInv , I assume you get 8200 records.
If you do ;
from tblCustody c
join tblPhoneInv p on c.deviceId = p.deviceId
I'd be willing to bet you get the full 43k records.
Tableau isn't importing these as individual little tables, it makes everything into one big flat table.
So if you have 3 records in tblPhoneInv for the same device Id, you will get three records in Tableau.
Using Level of Detail calculations for anything metric based is a good idea - Removing Duplicate Data with LOD Calculations | Tableau Software
Take a look at the 'Troubleshooting Joins' section here for a further explanation - http://onlinehelp.tableau.com/current/pro/desktop/en-us/help.htm#joining_tables.html#troubleshoot
You are exactly correct in your calculations. I'll take a look at the videos you supplied and see what I get from there.