What is the smallest viable angel investment to warehouse a deal?

The amount is not as important in warehouse deals, what’s more important is markups.
Let’s say that you invested 10k in warehouse deal, but now that position is up 30x. That’s a meaningful markup for your LPs that they would get at cost. And, if you can follow into deal or have pro-rata, that’s even better. 
You are going to need to set clean and consistent rules about the deals you warehouse because you can’t cherry-pick. So, for example, you can say, all deals in 2022 will get moved into the fund. You CAN NOT warehouse an SPV.