jrsc belt size doesnt make sense

[dragk20]

I have googled belt size for jr supercharger, k20a2 with ps and jmerc auto tensioner, ac spacers @ 9psi and it recomends 81.5"!!!! im using 74" @ 5si?! does that make sense? i figure 73"/73.5" for a smaller 9 psi pulley? or am i wrong?