i'd make it 2203, listOrder is generated internally in increments of 100 for all items that appear in lists, starting from the top to the bottom.
rifle clip is the 22nd item listed in the ruleset, hence it's listorder is 2200. you want it to appear after this in the list, so you set it to a higher number, the higher the number, the lower on the list it goes. 2201 is the sniper rifle, 2202 is it's standard clip, 2203 is the next open slot between rifle clip and heavy cannon (2300)
this also has some future proofing built in, so if you have 2 or more mods that define an item using the same listorder, it will automatically assign the next available listorder slot according to ruleset load order. so if you defined your clip as 2200, but listed it after xcom1ruleset and sniperRifle, it would automagically get pushed down to 2203 anyway.