velocity stacks

[njv]

hi
what is a good distance between the top of a velocity stack on a carb and the inside of the top of the air filter.

i only have 45 mm tall air cleaners and have ordered some 25mm tall (short by most peoples standards) velocity stacks .

no room for taller air cleaners in my typ 3 but i know the stacks can do wonders for hp.

but with only 20mm of room above them am i defeating the object?

love to know what you all think.