If you just want to see what cards you might need to get for a deck idea, just mark it as a built deck for bit, check it out, and then mark it as a deck idea again.
I just tried this and I'm not sure I'm being clear enough in what I'm trying to do.
As an example, I have a single copy of Koma, Cosmos Serpent. I have it in my inventory and I have it in a built deck where I'm running it as a commander for commander format. So since it's a built deck and I only have 1 copy of it, I don't have another card to actually put it in another deck if I wanted to. If I now put together a new deck idea and have Koma in that deck idea, it still shows that I have 1 available. If I were using that deck idea as a guide for singles I need to order, I wouldn't order another Koma because it shows I have 1 available even though I don't really have one available. I tried changing the idea to a built deck but it still shows the same.
I guess what I'm trying to figure out is if there is a setting or option that would make it show zero Koma available in a deck idea since I only have one in my inventory and it's already listed in a built deck.