FWIW
While there are many exceptions here are a couple VERY BASIC general things I use for determining how much current to put my corals in.
1. Type of coral...SPS, LPS, Soft, Other
2. Shape of coral...branching, encrusting, disk, other
3. Feeding requirements...filter, light, target, other
4. Overall size of coral
5. My personal expierence...sorry
1. The type of coral you have is important when considering current. Ask yourself the question " Can to much current damage the coral?" for example a branching coral like your colt recieving even strong current, likely will not get damged. LPS corals like an open brain, hammer, frogspawn or your plate where a hard "skeleton" and a large meaty soft polyp could separate from each other in strong current. This potentially could cause lethal damage. Cynarina (jelly brains) are very delicate and will not survive in anything but low current areas.
2. The shape of a coral is important as well. A branching coral provides less resistance to flow by definition, than say one with a disk that forces water around it. So a leather coral needs some current to keep the detris from settling in its disk but not so much as to prevent the polyps from fully extending. Encrusting corals like star polyps, gorgonian, montipora etc like strong current and will thrive with it.
3. Feeding requirements need to be considered when you have filter feeding corals. Without sufficient water flow to strain food from they may slowly starve. If target feeding is necessary (like with sun polyps) too much current may make this very difficult.
4.Overall size is common sense. While a 10" tall colt may survive direct strong current , a new 1" frag might simply blow away.
NM was correct...The plate should go in the sand bed. My guess is your lighting will not support it there and thats why the LFS told you to put it at the top. Because it extends polyps to feed it needs moderate flow but because it is also attached to a hard "skeleton" the flow must be either strong AND indirect or moderate enough as not to cause tissue damage...get the idea?
Sorry if this seems confusing.
SiF