Your 70-80 % matches with my 65 overall yield. I had bad luck with your procedure and this was the only run that finished without any accident, I had some hazards, one time the flask fall from my hands to the floor, and other time the refluxer was stoppered with crystallized ammonium bicarbonate, so the methanol was boiling under pressure, when my friend removed the refluxer... well, I still have the roof stained, but no more serious consequences. This is a important question to considere in CTH with ammonium formate under reflux, it is formed ammonium carbonate in the refluxer and it must be watch over.
The reaction seems anyway a bit capricious, and yields seem to vary considerably, for example a friend has recently performed it and reported about a 40 % overall yield.