New Column based upon other two columns

I am at a bit of a loss on how best to proceed. I need to do a complex calculation based on values from two different tables. I thought I create variables and then do my calculation from there, but I ran into an issue where I could use my SUMX function and join to one table, but not another. 

Here's the scenario:

Customer A has three contracts for $500,000 each. They total $1.5 million. At a row-level context, there's a calculation we do based on contract number that is similar to income tax rates. A liability amount is calculated based on a progression. The first $100,000 is excluded, then the next $250,000 is covered at 80% and anything over $350,000 is excluded.  The formula for that is done and working. The twist on all of this is that there is a policy limit of $1 million. We are not liable for anything in excess of that at a customer level. In this example the total liability is capped at $1 million. The customer level context is obviously a step higher than the contract level.

My data looks something like this:

Customer (table)

CustomerKey (PK), CustomerCode, CustomerName

Contract (table)

ContractKey (PK), CustomerKey (FK), ContractNumber, ContractAmount

Insurance (table)

PolicyKey (PK), InsuranceType, PolicyYear, MinimumAmount, MaximumAmount, Exposure, AppliedPct

I have a one-to-many join from customer to contract. The insurance table is not joined to anything and behaves more like a lookup. We have a compound key (insurance type + policy year) that serves as a join to the contract table.

Sample data looks like this:

Customer

Customer Key 501

CustomerCode 49113

Contract

ContractKey 34

ContractNumber 480, 481, 482,

CustomerKey 501

ContractAmount $500,000, $500,000, $500,000

Insurance

CompoundKey: DefIns 17-18

Type: DefIns

ApplicedPct: 1.0

CompoundKey: DefIns 17-18,

Type: DefIns,

PolicyYear 17-18,

AppliedPct 0.8

CompoundKey: DefIns 17-18,

Type: DefIns,

PolicyYear 17-18,

AppliedPct 0.8

I started writing out something like this: Amount = var DefIns = SUMX(FILTER('Insurance', 'Insurance'[CompoundKey] = 'Contract'[CompoundKey] . . .

Only half of the join works.

Finally, is this a calculation that PowerBI can do? Or should this be done in SQL?

Thanks.