If I am understanding you correctly, you want to write a calculation each distinct work order level. This is very simple in Tableau--if it was the difference of the two values, you'd just write a calculated field like:
[Actual Hours]-[Standard Hours] (not sure if this is the correct math; this is simply an example)
Then, simply place your order number field on the Rows shelf, and the calculation next to it, or on the Text shelf and it will do the grouping for you (it will aggregate at the level of each individual Order Number). I hope this is helpful; if not, it is possible I just do not understand the problem properly.