Are you looking for your "score" or the perks you earn for that sortie??
I am not sure how the whole score system works....might want to ask one of the folks listed every month on the home page. (like SHawk)
But if its perks you are asking about, then thats the formula for them (i think)
Just think of it like this the greatest number of perks you can earn while flying a low eny plane, like a P51D is by killing another low eny plane like the P51D. But in this case you will only earn whatever the perk mult. is at that time, usually around .75 to 1.5. Now think of it this way, if you were flying a P40 or a Hurri Mk1 (my fav) and shoot down that same P51 you could get upwards of 20 perks for that one kill. Hope this helps out a bit