Boom! Figured it out...
SELECT tblCurrencies.CurrencyCode,
tmp1.SumPay,
tmp1.SumPayUSD,
tmp2.SumRec,
tmp2.SumRecUSD,
((SumPayUSD + SumRecUSD)/(SELECT Sum(tblTransactions.USDAmount) FROM tblTransactions)) AS Proportion
FROM (tblCurrencies....
Completely forgot I...