To get the total of the same field over multiple records, you have two options, the Sum Function or a "total of" summary field. In similar manner, you have a count function and a "count of" summary field to count the records if that should be needed.
I'll call your "columns" to be totaled Field1, Field2, Field3 for this example.
define 3 more fields, sTotalFld1, sTotalfld2, sTotalFld3 as summary fields. Specify sTotalFld1 as the total of Field1, sTotalfld2 as the total of Field2 and so forth.
Define a calculation field, conversionRate, as:
stotalFld1 / ( sTotalfld2 + sTotalfld3 )
This is just the most basic solution to what you describe. The Summary fields will compute the totals of all records currently present in your found set for this table. If you perform a find to pull up a different set of records, these fields will display a different set of totals and a different result in the converstionRate field.
If you want to compute these values for groups of records to get different sub totals and different conversion rates, this can also be done if you sort your records, add sub summary layout parts and use the getSummary function.
Note: it would be a good idea to look up aggregate functions (sum, count, etc), summary fields, and getSummary in FileMaker help to learn more about them.
Sorted it, thanks for your help! :D