Hi Loomis,
Congratulations on making a good choice for your topper. 2" of 24 ILD is usually a fairly “safe” choice for people who are in an average weight range and where there is already some softness in the comfort layers of their mattress. it seems to me that you did well 
[quote]I’m 6’5" and 175 pounds…long and skinny…so I’m not going to sink in like alot of other people.
I might be able to increase the height of the topper and not have to worry about going up in ILD.[/quote]
This would depend on how your weight was distributed, your body dimensions (generally width of shoulders vs waist in men), on your sleeping positions (side sleepers often need thicker than back sleepers),and perhaps most of all on the firmness of the mattress that the topper is being used on (less soft foam in the comfort layer of the mattress means it may be “safer” to use a thicker topper).
I would tend to use the topper guidelines and make your choice based on whether you need a “little”, a “fair bit”, or a “lot” of extra softness compared to sleeping on the mattress alone. The relative firmness of your current mattress compared to your upstairs mattress would also be a good guideline (firmer mattresses may do better with a little thicker topper). It seems that 24 ILD though is a good choice for you which at least takes care of one of the two main variables (ILD and thickness).
Phoenix