Alignment turning plates - need advice

[70chall440]

Before I pull the trigger here, I need a little bit more information. So there 2 styles of turning plates, those that just rotate and those that move in all directions as well as rotate. The plates that just rotate have an index system on them wherein you can "zero" the wheel as well as degrees in both directions (I understand what this is for and how to use it) however the other plates do not seem to have any index or degree markings which leads me to believe you need to use another device to swing the wheel 20 degrees one way, then zero and move 20 in the other to obtain the number you are after. This is probably self explanatory but I just wanted to type it out and think through it (bear with me, it's a mental process). I have an alignment tool (bubbles) so this isn't an issue, I guess I am just trying to fully seat this in my mind. I appreciate your patience 

[brads70]

Those Gil smith one say you use a felt marker to set zero , the plates have 20-30 degs marks from what I read?
The longacre ones I have come with a adjustable zero tab . Same thing just a fancier way of doing it. 

[70chall440]

Gil Smith has 2 models, the one I want moves side to side as well as rotates. That one doesn't have any indexes on it. The other one that just rotates comes with an index and dry erase marker.