Rank Structure with permission so lower ranks cannot promote higher?

I have searched and cannot find this answer so I figured I would ask. This might be a little lengthy explaining, but I want to make sure I am clear.

I have Enlisted Ranks and Officer Ranks on my server. I want the officers to be able to promote the enlisted, but using a 1nd LT as an example, the only one below him would be a 2LT. I want him to be able to promote to that level but not higher. So a major can promote any enlisted and officer ranks to captain, which is right below major.

I feel that this deals with powers and I am not sure how to do it and am not totally sure where. I feel that it would be in group and I think that it would be something to the extent of giving each rank a power, say 2nd lt grant power of 20, 1lt grant power of 30, captain grant power of 40 and so forth. What I do not understand is max grant power. An example of this is Server admin has a grant power of 75. If I do a grant power of 30 and needed grant power of 30 for that rank, will it allow anything 30 or below to be granted and everything else grayed out? I want to run this by people before I start messing with a bunch of stuff that can screw up 2 years of me tweaking this server. Any input is welcome and thanks for reading.