I'm sorry, but without having the data and the database in front of me to help figure out what exactly is going on there, I'm afraid I'm at a loss. I could only assume your where clauses have to do with on which side your conversion calculation is occuring, pre- or post- data filtering, but that doesn't make a lot of sense even to me.
The only 'easy way' out I could see without digging into your structure and data is (*twitch*) to create an actual numeric column next to the other field, and run an update through your table to push numbers only into the field (CASE WHEN ISNUMERIC(@field) = 1 THEN @field ELSE NULL END), and use THAT field for these conversions.
Sorry I can't help you more on this one.
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