Add bleed+trim given whole dimensions for bleed+trim?

Hello,

A publisher has given me whole dimensions for bleed+trim for a printing project I am working on. If it matters, the file is a pdf that I am currently editing in Illustrator to add bleed+trim marks. For example, the bleed is 3.5" x 2" (WxH) like in the example image below.

From my understanding, bleed in illustrator is added to each side. Is my only choice to get the width of my doc, subtract it from bleed dimensions, divide by 2, and add to each side, like (bleedWidth-docWith)/2 and add this value to each side? Or is there a way to just enter my bleed as the whole dimension 3.5" x 2". 

Same goes for trim marks.

Thanks for reading!