You raise good question because deck mounted roman tub valves can cause troubles for people installing these IF they did not plan on DEPTH involved.
In this case, I would just place the spout approximately where you think it should go...meaning you will just hold it in place over the place you intend to install....then you need to evaluate this. There is no set standard!! Just make sure you leave enough room around that tub spout that you can secure and connect it from underneath as needed WITHOUT the nuts/washers running into the tub itself!!!
You want to maximize fill while not intruding on space/comfort of person(s) using tub....this can be tricky!!
I have even had a few customers that simply purchased the wrong valve for the job...needed higher arch and deeper roman tub spout...and so had to return or I had to install at the tub itself (drill holes in tub top).
Are you setting this tub in a bed of mortar or similar...? The instructions will recommend this and so do I. Read up....