standard deviaation field

Dear Expert ,

I want to use formula for calculating T distribution in maintenance task list for calibration plan. But in that

I am not able to use standard deviation field in that. For this I am trying to define parameter in customization

but I do not found corresponding field name and table name in database.

I want to use field standard deviation which we can see under further detail tab of inspection lot.

Please suggest .

Regards,

Prasad