Hops contain Alpha Acid and a couple of thousand other interesting bits - but in brewing bitterness is mainly concerned with AA. Magnum is a reasonably high AA hop, the one my local is selling has 14.8% AA.
Bitterness can be equated to the amount of Alpha Acid in solution, roughly 1mg/L is 1IBU (1/1000g/L). This isn't exact but will do for a working assumption.
The other really important thing to remember is that IBU's are measured in the finished beer (in the glass).
Now this is where it starts to get complicated.
Alpha acid is insoluble, when we boil Alpha Acid some of it gets rearranged into Isomerised Alpha acid that is soluble.
Under ideal conditions about one third of the Alpha Acid is Isomerised, there are lots of different factors that affect the amount that converts, some of the main ones are
Temperature - Hotter more and faster conversion.
Time - Longer boils have higher conversion (up to ridiculously long times around 4-5 hours)
Boil Vigor - A boil that is jumping around and turning over quickly will convert more than a limp simmer.
Wort Gravity - The higher the gravity the less conversion
Hop concentration - the more you add or the more concentrated, the less converts (as a %)
pH - The higher the pH the more conversion but the bitterness tastes harsher and rougher, lower pH gives nicer tasting beer.
There are more - but add them all together and you get Utilisation, basically how much of the AA you add turns into IsoAlpha.
Just to make things even more interesting, there is an upper limit for IsoAA solubility at 20oC its about 100 IBU.
Iso Alpha is also really sticky, it sticks to yeast, trub, filters, fermenter walls, the glass..., what we have at the end of the boil can fall by 30% by the time it ends up in the glass.
My personal opinion is that the best hop calculations are really an educated guess, unless you have a pretty good lab the best answer is a Goldilocks not enough, too much and just right! Sure use a calculator but know the answer is an approximation and for gods sake don't quote decimal places that's just silly, even +/-5 IBU is pretty brave.
In a home brew getting 25% Utilisation would be pretty good.
If you have 13g of Magnum with an AA% of say 14.8% you have 13*14.8%=1.924g of AA (1924mg)
If you got 25% Isomerisation that's 1924*25%=481mg
If that was diluted into 24L, 481/24=20mg/L or 20IBU At Most. doing a 45 minute boil, at high gravity... I doubt you will get that much.
Sure your late hops will add some, but not I suspect another 28IBU's.
On the second question.
The hops added early (the Magnum) loose almost all of their aroma, its volatile and boils off, most if not all of the hop taste is also broken down in the boil.
From that we can work out that we get more aroma from the latest/coolest hop additions, we also get more taste from the later hop additions.
Where it gets a bit ticklish is deciding when to add the late hops to get the character we want. For taste hops it is generally thought that somewhere between 10-20 minutes is best. You get measurably more flavour components at 20 minutes from the end, but it isn't the same taste a lot of people think its rougher more abrasive flavour than you would get from the same hops added at 10 minutes and at 20 minutes any aroma will be lost, at 10 some is retained.
The right answer is the one that works for you, you need to brew any beer a couple of times taste critically and experiment until you get the beer you want.
Have lots of fun!
Mark
