Ok I've finally solved the problem with the Bittrex fees and 8dp truncation etc. BTCBTC
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In summary: Since Bittrex truncates to 8.dp. any calculations taken off Bittrex's unit figures will be incorrect. Better to rely on Bittrex's own reported Price and CommissionPaid fields and work back from there to get unit figures (if you want to see more than 8 d.p).
Thanks for this, following Bittrex numbers my math was not adding up either. I ended up starting off off the amount received in the wallet for the trade and the Units Filled.