Hi Logan,
Very interesting problem you have here, as you don't have relationships between tables.
Lets first create measure AvgAllocPct in the Amounts table with following DAX formula:
=Average(Ratios[AllocPct])
I need this calculated measure to deal with cases when more than one row with AllocPct could be returned.
Now lets create our calculated measure with following DAX formula:
=CALCULATE(SUMX(Amount, Amount[Amount] * Amount[AvgAllocPct](FILTER(Ratios, Ratios[AllocCode] = Amount[AllocCode]))))
Could you please try this and see if that works for you? Please let me know - I would be very interested to hear if that works as you expected.
Regards,
Vidas Matelis