If you have an electronic mp gauge turn on the master and note the mp on your gauge prior to start up. Do a normal preflight and take off. Note the mp on the same gauge just as you lift off. Vans FAB looses an average (on 4 test planes) about .8 of an inch of mp (comparing static to t/o mp) through the induction under these conditions. Anything less than 1" loss is considered good.
I did a LOT of induction system testing with my carbed James cowl system back in 06 due to induction restrictions (1.8" mp loss)---ended up making my own carb box--I now loose .6" mp under the above conditions.
BTW, Will James redesigned his carb box to reduce the turbulance and associated induction loss.
Hope this helps,
Cheers,
db