Calculating hop replacement weights for different boil times

A question about utilisation calculations and hop replacements...

Here's the scenario... I've got a basic grain bill and I want to vary the hops each time I create the beer. Obviously, different hops mean different alpha/beta/essential oils.

Let's also say, for the sake of argument, that my hop additions would go:

• Bittering / Target / 20g / 12.0% AA / 60 mins
• 1st addition / Target / 20g / 12.0% AA / 20 mins
• 2nd addition / Target / 20g / 12.0% AA / 5 mins

Now let's say I wanted to change all of these additions to, say, Simcoe at 14.0% but I don't want the beer to be any more or less bitter or flavoursome... just to carry a different hop profile.

With my bittering hops what I would generally do is bang out a quick calculation like this:

20g x 12% AA = 240

then 240 / 14% = about 17g Simcoe.

Whether this is actually the correct way to do it, I'm not entirely sure but so far it has worked for me. However, when it comes to the later additions where we're dealing with comparitively little bittering potential from the hops and much more flavour/aroma I'm sure this calculation wouldn't hold up and I might be inadvertently changing the flavour profile.

I've been doing it this way up until now but I want to start working with very low AA hops like Crystal and my usual calculation I'm sure will upset the flavour balance significantly.

How would other people deal with this calculation?