hi all not new to i.c.e but new to building slot boxes help

hi all i have a generally good level of knowledge about installing ice in general i have made a few different things from boxes(sealed) to dashboard head unit consoles for din2 head units .

im new to building ported and t line and don't quit get how u get the measurements for these designs ..

for instance im looking to build a single slotted (center) twin 12" sub box using 2 mutantmt1207 subs but im struggling to find the info needed to make the box

this is sort of what i want but with the face on a angle not square like this one

any help would be much app thanks mick