There are hop substitutions listed that give hops with similar dry hop character. I have not seen a calculation for it, I would assume a like for like weight of a suitable substitute hop.
Not a huge amount of bitterness is extracted during dry hopping... as the alpha acids are isomerised during the boil. You could do a pro-rata based on the %AA (for example 10% more hops if the %AA is 10% lower), although I don't think it would really achieve anything and may result in an over or under addition. There are a lot of other volatile molecules involved in dry hopping that are lost in the boil so Alpha Acids aren't the only factor.
There is quite a unique character to many hops when dry hopped so the exact flavours aren't a direct swap and some hops are more forgiving than others in terms of what quantity is palatable. (For example I find Simcoe can be overpowering)
