I have the following calculated field in the "Build Query" diagram:
RetenRate_CALC (TIMEPERIOD field not shown, but logic works) =
IF((COHORT_SUMS.TIMEPERIOD=2 OR
COHORT_SUMS.TIMEPERIOD=3 OR
COHORT_SUMS.TIMEPERIOD=4)
AND
COHORT_SUMS.Sum_ENR_UG_SAME_UNIT>0,
((COHORT_SUMS.Sum_ENR_UG_SAME_UNIT/COHORT_SUMS.COHORT_TOT)* 100), '')
Which produces the following results:
I was trying to round the RetenRat_CALC field to two decimal places. Nothing was working, so I widened the column of the calculated field, where it seems I was trying to ROUND() a string field where I would have thought (because of the calculations) it would have been numeric.
Therefore, what is the best way to:

convert a string field to numeric; is transform/cleans the only way to accomplish this?
 I need the 0 or <0 fields to be null. 
Does doublesingle quotes nullify a field or is there a better way?

Is there a any difference between the functions in the "create calculated field" in the query builder verses the functions available in transform/cleans?
Thank you for any advice you can offer!