My first thought would be the flat portion. But possibly, you should add a "bit" from the angle.. say... 33% of the width of it... maybe. But I wouldn't expect to use the total width.
PDX 4.100, 2.150 & 1.600 3Sixty.2
Seas P18RNX & 27TAFNC/G (7" Poly & Neo Aluminum)
10w6v2 with Mr. Marv Custom Enclosure