Design Question with Conditional Dropdowns

I may be going about this the wrong way and need to start from scratch, but I am not sure. 

I have many projects types, let's call them Project Type 1-10.
Each project type can have a number of options, say Options 1-5.
Each option has a number of labor positions associated with it, and each labor position has a specific rate.

I would like to create an invoice and be able to do the following.
Pick a project type. Then pick an option. That will populate the labor positions and I can just choose one. This I've accomplished using cascading dropdowns.

However, I don't want to be limited to picking just one labor position. I want to have say 10 fields where I can keep choosing additional labor positions (as I pick the labor position, the rate can be populated from a lookup field. This rate is really what I need for future calculations).

I suspect I am one table short, and need to create a many to many relationship somewhere in here, but I can't wrap my head around it. Or maybe there is a much more elegant solution I am not thinking of?

Thanks for any help!

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Mike Tamoush
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